204 DR. COHN, ON THE DEVELOPMENT OF

202 DR. COHN, ON THE DEVELOPMENT OF

these bodies were animal existences, is perhaps indubitable,from their phosphorescence; but in what connexion theystand with respect to the JVoctiluccB, associated with whichthey occurred in large number, cannot at present be deter-mined."

UNTERSUCHUNGEN ueber die ENTWICKLUNGS-GESCHICHTE derMIKROSKOPISCHEN ALGEN und PILZE. Von Dr. F. COHN.(Researches on the Development of the Microscopic Algaeand Fungi. By Dr. F. Cohn ; pp. ] 53 ; 6 Plates. Bonn,1854.

THE importance of the study of unicellular organisms, asleading in the most ready and complete way to a knowledgeof the " cell," the foundation of all scientific acquaintancewith the real nature of plant life is so obvious, as to haveattracted a great number of followers.

The fresh-water Algae, especially, afford abundant andreadily attainable materials for this study, and have, conse-quently, formed the subjects of numerous writings. Amongstthose who have distinguished themselves in this field, thename of Dr. Cohn will ever be held in deserved honour.He has, for many years, as he says, devoted himself to thisstudy; and, especially, to the remarkable propagation ofmost of these Algae by means of motile cells (swarm-spores) ;a mode of reproduction which has been observed in most ofthe fresh-water species. The same phenomenon has also at-tracted the attention of numerous other observers, and been thesubject of several memoirs. Among the more important ofthese, exclusive of Dr. Cohn's, may be noticed the observationsrespecting it contained in Dr. Braun's remarkable work on' Rejuvenescence in Nature,' of which a translation by Mr.Henfrey has lately been published by the Ray Society; andthe ' Recherches sur les Zoospores des Algues et les Anthe-ridies des Cryptogames' of M. Thuret. The appearance ofthese independent memoirs appears to have turned Dr. Cohnfrom his original intention of publishing a special treatise onthe 'Swarm-cells of the Algae,' and to have decided himmerely to give separate essays on those points appearing tohim to demand further attention ; his former monograph onthe ' Development of Chlamydococcus {Protococcus) pluvialisjof which an abstract has also been published by the RaySociety with figures; and his Memoir in Siebold andKolliker's Zeitschrift f. wiss. Zool. ' On a new genus from thefamily of the Volvocina,' of which a translation has appearedin the 'Annals of Nat. History,' 2nd series, Vol. x., p. 321,

MICROSCOPIC ALG.E AND FUNGI. 203

are the more important of these essays that have yet appeared,and should be attentively studied by all who are desirous ofbecoming acquainted with the matter.

The present work is, in fact, a collection of shorter essayshaving more or less direct reference to the same subject, andwill be found to contain a vast amount of important andinteresting information.

The subjects treated of are—

1. Ou the relation of the microscopic Fungi to the microscopic Algol.2. On Chytridium, and some allied genera.3. Observations on Oonium perforate, and the Volvocnia in general,4. On the propagation of Hydrodictyon utriailatum, together with

some remarks on " swarm-cells " in general.5. On the germination of Zygnema and Anabcena.

These papers are illustrated by figures ofAn thophysa Miillnri.Zooylwa (vibrio) termo.Spirulina plicatilis (Spirochct'tr, p.,

Bhr.)Spirulina Jenneri.Synedra putrida, n. s.OMamydomonas liyalma (Poh/toma

uvella, Ehr.)Chytridiwm gldbosum, A. Braun.

Zygnema stellinum.Closterium Lumila.Mougeotia genuflexa ?Anahmna intricata ?Achtya capitulifera.Ohlamydococcus pluvialis.Gonium perforate.Chlamydomonas pulviscv.lus.Hydrodictyon utriculatuin.

Peronium aviculare, Cohn. ' (Edogonium capiUare.AcMyaprolifera. Cladophora glomerata (monstrousSpirogyra nitida. spores of ).

1. On the relations of the microscopic Fungi to the micro-scopic Algae.

The result of his study of the lowest forms has led Dr.Cohn to the conclusion, that no sufficient reasons derived frommorphological and developmental considerations exist for theseparation of the Algae from the Fungi. Though the distinc-tion between the Thallophytes and Cormophytes of Endlicheris sufficiently definite, the three classes into which the formerhave, since Linnaeus, been subdivided, viz., the Algae, Fungi,and Lichens, are by no means so well defined as, for instance,the Mosses from the Ferns, or the Equisetaceae from theLycopodiaceae. It would seem as if the multitudinous classof Thallophytes constituted, organologically, but a singleindivisible kingdom, and that the above three provinceswere characterized merely by the more or less developed, and,it must be confessed, widely different forms, and in no wayby any intrinsic difference of type in the vegetative or repro-ductive organization.

The Algae, as usually understood, like most other plants,are capable of appropriating, by an innate power, the materials


requisite for the maintenance of their organism, from theelements carbon, oxygen, hydrogen, and nitrogen, and someoxides and salts, which are afforded to them in the surround-ing medium, in the form of carbonic acid, ammonia, andwater ; they do not, therefore, require any organic nutriment,but are enabled to vegetate in pure water ; they can decomposethe water or the carbonic acid, and evolve oxygen in thesunlight, acquiring at the same time a green or red colour,from the development of chlorophyll or some analogouscolouring matter. The Fungi, on the other hand, likeanimals and most parasitic plants, have not the power ofspontaneously producing, from inorganic nutriment, thematerials requisite for the maintenance of their vital pro-cesses—these must be afforded to them in the form of alreadyorganized compounds; they cannot, therefore, flourish wherethis nutriment is not afforded to them, either in a living or ina dead and decaying organism, or at any rate in water inwhich a considerable amount of organic matter is not dissolved,as in an infusion ; they evolve no oxygen, and do not becomegreen in the light. Partly, from the latter circumstance,Nageli distinguishes the Fungi from the Algse by the want ofchlorophyll, or some analogous colouring matter. But thatthe presence or absence of chlorophyll affords no sufficientcharacter to distinguish the one class from the other, is suffi-ciently obvious, when we consider the variableness exhibitedin this respect, in several classes of plants, especially in thoseof parasitic habits, many of which, it is true, are colourless,but others, such as the Santalaceae, Rhinanthaceae, and Loran-thaceae, have chlorophyll in abundance ; whilst others again,not of parasitic nature, or not known to be so, as many of theOrchids, are colourless. The presence also, of chlorophyll inmany of the ' Protozoa,' as Hydra viridis, Bursaria viridis,&c, is sufficient to indicate the uncertainty of any characterthence derived. The circumstance of the plants growing inwater, or in the air, has been employed to distinguish theAlgae from the Fungi—it being stated that the former inhabitwater, and that the latter flourish only in the air; but thatthis distinction is untenable is at once obvious, when it isremembered that it often happens that of species even in thesame genus, as Vaucheria, Ulothrix, Protococcus, &c., somevegetate in water and others in the air.

The difficulties which attend the separation of the lowerAlgce and Fungi have led several authors, as Kiitzing, topropose, for some of them, the erection of a group termedMycophycem, under which he includes those Thallophyteswhich agree with the rest of the Fungi in their vital con-


ditions, and consequent want of colour, and are only distin-guished from them by the unessential circumstance of theirliving in water. But a close investigation of the generaincluded under the above term, will show, beyond any doubt,that nearly all the forms of water-fungi atffe so closely alliedto algan genera, that, with the exception of their wantingcolour, scarcely even a generic distinction can be drawnbetween them, and much less one of family.

Thus the only difference between Hygrocrocis, a so-termedwater-fungus, and Leptothrix, consists in the circumstancethat the immotile filaments of the latter contain phycochrom,whilst in the other the contents are colourless. But whilstLeptothrix is, perhaps, inseparable from some forms ofOscillaria, notwithstanding the motility of the latter, so inthe same way do the colourless immotile filaments of thegenus Beggiatoa come under the class of water-fungi, close tothe various forms of Hygrocrocis. It has already beenremarked by Nageli that the yeast-fungus corresponds inform and mode of germination with the algan genus Exococcus,Sarcina is morphologically identical with Chroococcus, andthat, in its vegetative and reproductive condition, Achlyacorresponds with Valonia or Bryopsis.

With respect to the genus Stereonema, Kutz., Dr. Colinproceeds to show that the filamentous growth so named, isnot an Alga or Fungus at all, nor in fact any kind of inde-pendent organism, but that they are the stems of an infusorium—the Anthophysa Miilleri, Bory.

The bunches of apparent spores at the extremities of thesefilaments are regarded by Dr. Cohn, though apparently notwithout hesitation, as identical with the Uvella uva of Ehren-berg; and, consequently, they are in his view to be lookedupon as belonging to the animal kingdom, for he seems, likemany other German observers, still to adhere to the exclusivelyanimal nature of many of the Ehrenbergian Monadina.

The loss thus inflicted upon the vegetable kingdom by Dr.Cohn, is, however, compensated by the addition to it of partat least of the Vibrionia. The animal nature of these minutecreatures has, hitherto, never been disputed; but Dr. Cohnsees reason, and affords what appear to be good grounds forit, to believe that several forms of the Vibrionia may becertainly shown to belong to the vegetable kingdom.

The form which seems to have constituted the principalsubject of Dr. Cohn's researches in this respect, is that knownas Vibrio Hneola, Ehr., but which was separated from the otherVibriones by Dujardin, under the name of Bacterium termo.

The result of his investigations is, that the corpuscles of


Bacterium termo, Duj. {Vibrio lineola, Ehr.), represent thedevelopmental condition of a p lant ; that they .are, in fact, theliberated, self-motile cells (swarm-spores) of a Mycophycean,closely allied, morphologically, with Jfalmella and Tetraspora.H e has been unable, however, as yet to discern any motilecilia in them. He proposes a new name for this form—Zooglcea, with these characters.

Cellulifi minimaj, bacilliformes, hyaliiue, gelatiua hyalina in massasmucosas globosas, uvasfonn.es, mox niembranaceas consociatai, dein singulaselapsaj, per aquam vacillantes.

Syn. FcdmeUa Infusumum, Ehr.; Micraloa teres, V. Flotow; Bacteriumtermo, Duj.; Vibrio lineola, Ehr.

The general results of his researches on the subject of theVibrionia are thus summed up.

1. The Vibrionia all appear to belong to the vegetable kingdom,since they exhibit an immediate, close relationship with manifestAlgce.

2. From their want of colour, and their occurrence in putrifyinginfusions, they belong to the group of Mycophycecx.

3. Bacterium termo is the motile swarming form of a genus (Zooglcea)closely allied to Palmella and Tetraspora.

4. Spirochade plicutilis belongs to the genus Spirulina.5. The elongated, motionless vibriones (V. bacillus, &c.) are allied to

the more delicate forms of Jieggiatoa (Oscillaria).(!. The shorter Vibriones and Spirilli correspond, in form and in

their movement, with the Oscillarue and Spirulince; but nodefinite opinion can be given as to their true nature.

The relationship of the Oscillarise with the Vibrionia wasnoticed even by the earliest observers. Thus O. F . Miillertermed a Spirulina, Vibrio serpens; nor has the analogybetween them been overlooked by Ehrenberg and Perty.

Dr. Cohn goes on to describe the great resemblance betweenEhrenberg's Monas prodigiosa, the cause of the phenomenontermed " blood in bread," and Bacterium or Zooglcea termo.The main difference between the two consisting in the shapeof the former, which is more spherical or ovoid than bacilli-form, and its purple-red colour.* Allied with this are themasses of Vibriones, which, according to Mitscherlich'sobservation, appear on rotten potatoes, as a kind of ferment,and have the power of dissolving cellulose (Monatsb. d. B.Ak., March 1850). All these forms appear to be closelyallied to Zooglcea termo, if not generically identical with it.

According to Cohn there is not the slightest differencebetweenPolytomauvella, Ehr., and Chlamydontonaspulvisculus.

The general summary of his inquiries may be thus stated—That most of the Mycophycece, agree in family, and even in

* Upon this subject vide Fresenius, ' Beitriige zur Mycologie.' Part I.


genus with certain fresh-water Algae. Whence it follows thatthat class in general is not a natural group, but an assemblageof plants of various natural families and genera, joinedtogether by a single artificial character.

In this way the Cryptococcacese will be arranged under thePalmellaceae; the Septomiteae, under the Oscillarieae andSeptotrichese ; the Saprolegnieae under the Vaucherieae ; somegenera would be abolished altogether, such as Hygrocrods,whose species must be regarded as colourless species ofLeptothrix, and those of Beggiatoa as colourless Oscillarice:and as Spirochcete, which is identical with Spirulina plicatilis;Vibrio bacillus would probably be termed Oscillaria bacillus,&c.

2. Cohn's observations on the subject of Chytridium andsome allied genera are of particular interest in several respects,and especially with reference to the important matter of theconnexion between parasitic Fungi and certain diseased condi-tions in plants. The extraordinary prevalence, of late years, ofepidemic diseases attacking nearly all cultivated plants, andmany of which have been observed to be accompanied withthe development of Fungi, renders the determination of thetrue relations of the one to the other a point of extremeinterest and importance, not only in a scientific, but also in aneconomical sense. To the ancient and well-known pests of' rust' and ' smut' have been added, it may be said, withina few years, the more destructive potato—and vine—cisease ;but otlier important cultivated plants, as the olive—orange—beet-root, as well as timber-trees, belonging to the Coniferousfamily especially, have also suffered in a similar way.

Disease of this kind has been by no means confined tocultivated plants—these epidemics are not limited to plantsuseful to mankind—their ravages may be witnessed amongthe useless and noxious members of the vegetable kingdom,to an almost equal extent with the highly-prized objects ofhuman cultivation.

A remarkable instance of this is afforded in the presentMemoir of Cohn. And his observations will go very far tos-olve any remaining doubts as to the true nature of therelations between the parasite and the disease. Inasmuch asthe victim and the destroyer are both plants of the simplestkind—in fact, unicellular algse, in which the whole process ofthe invasion and its effects is plainly submitted to the eye.The main question to be determined is, whether the fungus isto be regarded as the cause of the disease, or whether thedisease is, as it may be termed, the cause of the fungus. Inthe former case, the appearance of the Fungus or its spores


would be seen to precede the outbreak of any morbid pheno-menon, and in the latter, the reverse would be observable.Other influences of a more general, external, chemical,physical, or it may be, cosmical nature, must, doubtless,concur, to render the disease, however produced — trulyepidemic—but these are not now the subject of inquiry.

The decision of the question as above stated is obviouslyextremely difficult, if not wholly impossible, in the higherplants, owing to the complexity of their structure, and theinability we labour under of tracing microscopically the entirecourse of the disease, from its first appearance to its termina-tion.

An observation, therefore, which Cohn states, he made inthe year 1852, of a disease attacking unicellular plants, is ofthe greatest interest; he observed a sort of cpidemy to breakout among some Desmidete which he had kept in a flourish-ing condition for some time. The consequence was, that theClosteria, especially, were nearly all destroyed with greatrapidity. He discovered that the cause of this remarkableepidemic was a peculiar microscopic plant, which attacheditself to healthy Closteria, living at their expense, and con-suming or destroying their living contents, and thus killingsthem. The unicellular nature of the victim and of its de-troyer allowed the whole proceeding to be observed through-out all its stages ; and thus, at any rate, one certain fact wasestablished with respect to the significance of epiphytes inepidemic diseases.

The matter is thus described :—" In the beginning ofApril he observed upon the dead Closteria of every species,spherical vesicles of very various dimensions, the smallestbeing scarcely 1-300'", and the largest more than 1-50'" indiameteT. These vesicles were filled with opaque, fine-gra-nular but colourless contents. They were seated eithersingly, or in larger or smaller numbers upon the cell-wall;on some Closteria as many as twenty might be counted."The granular contents of the vesicles were seen to becomegradually transformed into motile spores or ' swarm-cells,"which escaped through openings or perhaps ruptures of theparent vesicle, and moved about very actively in the water bymeans of a single cilinm. When all had escaped, the vesiclewas left as a colourless, hyaline membrane.

The spores themselves resembled in shape the minutemonades, and equally resembled the 'swarm-cells' of Achlyaprolifera, but from which they differed in their very remark-able motility. From this, and from their correspondence inform and size, Cohn regards it as probable that these Chytri-

MICROSCOPIC ALGM AND FUNGL 209

dium-spores may be identical with Bodo saltans of Ehren-berg.

These ' swarm-cells' continued to move about in the wateruntil they Teach a situation suited for their further develop-ment,—that is to say, until they arrived at a new Closterium.And, what was especially remarked, they invariably attackeda •perfectly-healthy, briskly-vegetating individual, in which thegreen endochrom was in close opposition with the cell-wall.Having attained to its goal, the spore enters into a new stageof development—it becomes quiescent, affixes itself, and beginsto germinate; that is to say, the cilium disappears, and thespore, surrounded with a rigid membrane, assumes the aspectof a spherical, colourless cell, whose opaque nucleus is stilldistinctly recognizable, in close contact with the membraneof the Closterium.

The ' swarm-spore' now rapidly expands into a largevesicle. The nucleus undergoes a remarkable change ; itdisappears, and is replaced by a highly-refractive drop offluid, probably oil, which at first occupies the greater part ofthe cell; this drop divides into two, then into several, andultimately breaks up into numerous granules or droplets.When the vesicle has reached a diameter of about 1-100"',its contents are seen to be divided into two portions—adarker placed on the side at which the vesicle is attached, anda lighter at the outer periphery. The former, or dark portion,is produced from the metamorphosis of the nucleus, and con-sists of numerous larger or smaller granules. This darker orgranular part of the contents gradually increases at the expenseof the other, until at last the whole cell is filled uniformlywith it. When this has taken place, the reproduction com-mences, the granular contents of the vesicle being broken upinto a vast number of spores, by which the parent cell iscompletely filled, as is the case in Pilobolus and other Mu-corineae. In most "of the fresh-water Algse, however, thedevelopment of spores takes place in the protoplasma liningthe wall of the cell.

The parasitic growth thus described, belongs to the genusChytridium, instituted by A. Braun in his Work on ' Reju-venescence in Nature.'*

The most important point, however, is with respect to theinfluence exerted by the Chytridium, in the course of itsdevelopment upon the Closterium.

Wben the ' swarm-spore' has reached a certain size, thecontents of the Glosterium begin to exhibit a morbid change.

* Translated by A. Henfrey, and published by the Bay Society in avolume of Botanical and Physiological Memoirs, p. 185. A collection ofgreat value to the microscopical observer.

VOL. 111. P


The primordial utricle retreats from the wall and contracts,expelling the water from its interior. The peculiarly-arrangedChlorophyll is detached, and begins to be discoloured ; at thesame time, the two vacuoles, filled with granules at each end ofthe Closterium, disappear; ultimately, the cell appears colour-less and empty, merely some remains of the chlorophyllbeing left, in the form of an irregular contracted saccularmass or masses in the middle of it. That this morbid changeof the cell-contents is due solely and wholly to the influenceof the parasite, may be shown beyond all doubt. For thecontraction of the primordial sac begins at the spot where theCliytridium spore was attached ; and according as the parasitehas germinated at the middle of the frond, or at one or other ofthe ends, it is there that the contents first appear to suffer.

It is clear, therefore, that the Chytridivm is nourished atthe expense of the Closterium; and it may readily be con-ceived, that a parasite of this kind may produce a most de-structive epidemy among the millions of inhabitants in aglass of water, when it is considered that each Cliytridiummay contain 4,000 motile spores; each of which is capable ofdestroying a Closterium, and after a few hours may itselfreproduce the same number of spores.

As regards the mode in which the fatal influence of theCliytridium is exerted, it would seem to consist not simply inan endosmotic action through the walls of the two cells, butby means of a kind of radical fibres which insinuate them-selves into the Closterium, and which may probably be re-garded as equivalent to the mycelium of a fungus. Dr. A.Braun (1. c.) expressly describes these radical fibres, as theyappear to have occurred several times to Cohn's observation,who has given figures of what he saw. It would thenceseem, that the Chytridium-vesicle is attached to its organicbasis, not simply by adhesion, but by a myceloid tissue,which penetrates the membrane of the Closterium, and ramifiesin its interior.

The systematic position of this parasitic fungus, for such itmust obviously be regarded, unless all the Mycophycese aremerged in the Algan order, is clearly in that order, in thefamily of the Saprolegnieae, itself a section of the Vaucherieae.

With respect to the relations of the ' swarm-cells ' of theaquatic fungi to the monades. Just as the green ' swarm-cells ' of the true algce so clpsely resemble certain astomatousgreen infusoria, that it is impossible in many cases to deter-mine whether a doubtful form belong to the animal or to thevegetable kingdom ; so do the colourless monads in form andcolour precisely correspond with the colourless ' swarm-spores'of the Mycophycem. It is even probable that the number of

MICROSCOPIC AhGM AND FUNGI. 211

motile cilia is the same in each group ; for whilst all greenswarm-spores have at least two cilia, the colourless monadsand the swarm-cells of Chytridium certainly have but one;*in Achlya also, Pringsheim observed but one, which accordswith Cohn's own observation, though Thuret and De Baryassert positively that they have seen two.

To this it may be added, that a great part of the Infusoria,and in fact not merely of the lowest, but also of those standingcomparatively high in the scale, enter into a quiescent state,in which they lose all perceptible vestige of movement andorganization, and become invested with a rigid, perfectly-closed membrane ; apparently in precisely the same way thatvegetable swarm-cells, when germinating, secrete a toughcellulose-membrane. And in particular, do the true monadspresent a condition of encysting, in which they cannot be dis-tinguished from colourless, quiescent (fungus-) cells. Conse-quently, since neither the ' swarm-cells ' of the aquatic fungiexhibit any tenable criterion by which they can be distin-guished from monads, nor the encysted monads any by whichthey can be distinguished from the germinating aquatic fungi,there is no wonder that, in many cases, it is almost impossibleto determine to which category a given organism may belong.

Cohn, however, seems still indisposed to regard all theEhrenbergian monads as developmental states of fungi, as hehas convinced himself, he says, of the correctness of Ehren-berg's observation, that many, even very minute species, whenkept for some time in coloured water, take up particles ofindigo. This circumstance, Cohn regards as sufficient proofof the existence of a mouth, and consequently, of the animalnature of the organism ; in neither of which points, however,do we coincide with him.

3. The family of the Volvocina affords so many pointsof interest and importance, that the study of it may, to a cer-tain extent, be regarded as a fundamental basis for the know-ledge of microscopic organisms in general.

The vegetable nature of these organisms, first distinctlyrecognised by Siebold, and at present admitted by nearly allobservers, has already been copiously discussed in the pagesof this Journal, and the subject need not here be furtheradverted to.

The Volvocina, in general, may be said to consist of twoparts, a colourless hyaline investing cell (Hiillzelle), consist-ing of cellulose [?], and of green primordial cells.j The

* In Euglena, however, which is deeply coloured, there is but onecilium.

t For the explanation of these terms, vide Cohn, Prolococcus pluvialis(' Botanical and Physiological Memoirs,' published by the Ray Society,1853).

2


latter vary in number, from one to any number in the sameenvelope, being single, for instance, in Cklatnydococcus (Proto-coccus) and Chlamydommias, eight, as in Stephanosphmra, orinnumerable, as in Volvox. In either case, the primordialcells present the character of simple primordial sacs, whichare not immediately surrounded by any rigid cellulose mem-brane ; consisting merely of a fine-grained protoplasm, co-loured red or green by chlorophyll, or a peculiar oil, and oftenprolonged into mucoid filaments. The primordial cellsthemselves are produced peripherally into a colourless point,from which arise two vibratile filaments, which penetrate theinvesting sheath through two openings, and project into thesurrounding water. The reproduction is effected by the divi-sion of all, or, as in the case of Volvox, of some of the pri-mordial cells, which, in this case, come eventually to resemblethe parent organism; or the contents of the germinatingprimordial cell are more minutely subdivided into motilezoospores of much smaller size (termed microgonidid), whoseultimate destination is unknown ; the other or larger form aretermed macrogonidia. The latter, lastly, may assume the' quiescent' state, each primordial cell within the delicate' investing membrane' secreting around itself a second, moredense cellulose membrane, which is not perforated by themotile cilia, bat is in close opposition with the-primordial cell,just as in the common plant-cell the cellulose membraneencloses the primordial sac or utricle. In this, manifestlyvegetative, protococcusAUie condition, the cells may remaintorpid and without any movement for'almost any length ofdine, until their dormant energies are re-awakened by theaddition of water. It would even appear that in many cases,a previous desiccation is required, to render these winter-spores capable of germination. (These winter-spores, in dif-ferent states in Volvox, have been supposed by Mr. Busk tobe represented in the forms termed V. aureus, and V. stellatus.)

In speaking of Gonium, a genus referred by Cohn to theVolvocina, he remarks, that his own observations have led himto perceive that its structure is somewhat more complex thanit is usually described as being.

In the first place, the entire tablet is surrounded by aperfectly colourless, transparent envelope, presenting the formof a flattened spheroid, the axis of rotation being shorter thanthe other two. This envelope is not surrounded by any firmor cellulose coat, and appears to be simply gelatinous ormucoid; and it is, consequently, not very readily visible.As in other similar cases, it is best brought into view by theaddition of some colouring matter to the water. The greenglobules are simple or primordial cells—properly of an

MICROSCOPIC ALGJJE AND FUNGI. 21§

octagonal shape, or of a quadrate form with truncated angles ;and each of the sixteen cells is enclosed in a colourless,hyaline, delicate, but at the same time rigid membrane.Cohn, however, has hitherto been unable to determinewhether this tunic contain cellulose or not. The polygonalffonium-ce\\s, thus surrounded by a rigid membrane, are incontact with each other at the angles formed by the trans-parent, colourless wall, and these conjunctures have beenvariously understood by different writers. Such as " bridesblanches muquenses contractiles," of Turpin, and the" band-formed, tendril-like connecting tubes," of Ehrenberg.In other respects, the gonium-spores resemble the usual' swarm-spores,' like which, they contain chlorophyll vesicles,starch, &c. They also present a variable number of spacesor vacuoles, filled with water, which are sometimes sonumerous as to give the contents a frothy, vesicular aspect.With these variable vacuoles, however, must not be con-founded one, two, or three constant, sharply-defined vesicularspaces, situated close to the origin of the vibratile cilia. Thereproduction is effected in the usual way by the division,several times repeated, of the contents of the cells. Into theparticulars of which, resulting as it does in Gonium in theproduction of sixteen cells, Cohn enters at considerablelength. He then proceeds to discuss the relation of Goniumto other Volvocinae, and shows, notwithstanding some dis-crepancies, that it may properly be there placed.

After this, he returns to the subject of the vacuoles inthe Gonium-ceWs, and carefully distinguishes the vacuoles,formed apparently by watery secretions in the protoplasma,such as may be observed in all plant-cells, especiallywhen young, from others of a particular kind. In Gonium,he observed two or three vacuoles which disappeared andreappeared periodically or rhythmically at short intervals—and which he thence terms contractile vacuoles. He statesthat these vacuoles are always placed near the point ofinsertion of the vibratile cilia, and that they contractalternately at regular intervals of so many seconds. Con-tractile vesicles of precisely-similar kind have been observedin numerous infusoria, and were even seen by Ehrenberg longago, in the zoospores of Gonium and of Volvox, who entertainedthe extravagant notion that they were seminal vesicles. Re-ferring to these facts, Dr. Cohn, however, imagines that he wasthe first to discover the rhythmical nature of the contractions ofthese vacuoles, and enters at very great length, and in greatdetail, into an account of his observations. Not being aware,we presume, that he was anticipated by more than a year in

214 DEVELOPMENT OF MICROSCOPIC ALG E AND FUNGI.

this discovery, by Mr. Busk,* who noticed the fact of theexistence of rhythmical contractions in the single vacuole inthe zoospores of Volvox ylobator. But the most remarkablecircumstance connected with this is the apparently preciseresemblance of the phenomenon in the two cases. Mr. Buskstates, that in Volvox the contractions occur very regularly atintervals of 38'" to 41 ' " ; and Cohn's observations would showthat where two vacuoles exist in the same cell, the intervalfor each vacuole varies between 25'" and 48 '" ; and in a casewhere but one existed, as in Volvox, the intervals, by observa-tion, were 45,43, 45,42, 43, 40, 42, 41,41"'. The suddennessof the contraction, and the gradual expansion of the vacuole,are noticed by both observers ; in fact, the observations agree inevery particular, save only in the circumstance of the situationof the contractile vacuole or vacuoles. Cohn says that healways found them to be situated at the base, as it were, ofthe cilia—whilst Mr. Busk states that, " it may be situatedin any part of the zoospore, or not unfrequently in the base,or even in the midst of one or other of the bands of proto-plasm, connecting it with its neighbours."

This rhythmical contraction is, certainly, a remarkablephenomenon in the vegetable kingdom, and worthy of attentiveconsideration. It does not, as yet, appear to have beenobserved, except in the above two instances, but will, doubt-less, be found to exist in much more numerous cases.

4. The observations on Hydrodictyon do not appear tocontain more than a very good summary of what is known onthat subject, and are illustrated by good figures. We shallreturn at a future opportunity to this, perhaps, most interest-ing of all vegetable productions. Upon which will also befound very valuable information in A. Braun's work, abovereferred to. At the conclusion of this part of his work, Dr.Cohn propounds the following axiom :—That in the lowestforms (fresh-water algse) a motile, primordial cell, is at thesame time the seat of germination (swarm-spore), whilst inthe higher cryptogamia, it is the fertilizing organ (swarm-filament) ; on the other hand, in the latter a quiescent cell(spore) is the seat of germination, whilst in the highestphanerogams, an equivalent cell (pollen-cell) discharges thefunction of a fertilizing organ.

5. The observations contained in this part of the bookrelate to the germination of Zygnema and Anabaina. Theyare of considerable interest, and will, with the preceding,form the subject of a future notice or abstract.

* Transactions of the Microscopical Society. ' Quarterly Journal ofMicroscopical Science,' Vol. i., p. 31.

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204 DR. COHN, ON THE DEVELOPMENT OF