AWVYAIT ADOIOOZ Ong Digitized by the Internet Archive in 2010 with funding from University of Toronto http://www. archive.org/details/treatiseonzoologO1 lank A TREATISE ON ZOOLOGY A TREATISE ON ZOOLOGY Demy 8vo, Cloth, price 15s. net each ; or, in Paper Covers, price 12s. 6d. net each NOW READY Part Il. THE PORIFERA & COELENTERA By E. A. MINCHIN, M.A., Professor of Zoology at University College, London; G. HERBERT FOWLER, B.A., Ph.D., late Assistant Professor of Zoology at University College, London ; and GILBERT C. BOURNE, M.A., Fellow and Tutor of New College, Oxford. Part Ill. THE ECHINODERMA By F. A. BATHER, M.A., Assistant in the Geological Department of the British Museum; assisted by J. W. GREGORY, D.Sc.,~ late Assistant in the Geological Department of the British Museum, Professor of Geology in the University of Melbourne, and E. S. GOODRICH, M.A., Aldrichian Demonstrator of Anatomy in the University of Oxford. Part 1V. THE PLATYHELMIA, MESOZOA, AND NEMERTINI By W. BLAXLAND BENHAM, D.Sc. (Lond.), M.A, (Oxon.), Pro- fessor of Biology in the University of Otago, New Zealand ; formerly Aldrichian Demonstrator of Anatomy in the University of Oxford. AGENTS IN AMERICA THE MACMILLAN COMPANY 66 FirtH AVENUE, New York 9 Yy A TREATISE ON ZOOLOGY EDITED BY KH. RAY LANKESTER M.A., LL.D., F.R.S. HONORARY FELLOW OF EXETER COLLEGE, OXFORD ; CORRESPONDENT OF THE INSTITUTE OF FRANCE; DIRECTOR OF THE NATURAL HISTORY DEPARTMENTS OF THE BRITISH MUSEUM Part | INTRODUCTION AND PROTOZOA SECOND FASCICLE BY J. B. FARMER, D.Sc., F.R.S. PROFESSOR OF BOTANY, ROYAL COLLEGE OF SCIENCE, LONDON J. J. LISTER, M.A., FBS. FELLOW OF ST. JOHN'S COLLEGE, CAMBRIDGE E. A. MINCHIN, M.A. PROFESSOR OF ZOOLOGY, UNIVERSITY COLLEGE, LONDON AND S. J. HICKSON, F.R.S. PROFESSOR OF ZOOLOGY, OWENS COLLEGE, MANCHESTER LONDON : ADAM & CHARLES BLACK tt 1903 PREFACE TO SECOND FASCICLE OF PART I. INTRODUCTION AND PROTOZOA THE irregular publication of the parts of the Treatise on Zoology is the inevitable result of the fact that it is the work of a number of authors. I have determined not to allow Professor Minchin’s most timely and valuable treatise on the Sporozoa to lie by in the printers’ hands until the other sections of Part I. which logically precede it are ready for the press ; and with this I have been able to combine Dr. J. J. Lister’s section on Foraminifera (which contains much that is new and original), Professor Hickson’s section on Infusoria, and a section on the Structure of Animal and Vegetable Cells, with especial reference to the points which arise in the study of the Protozoa, by Professor Farmer. These four sections form the second fascicle of the First Part of this treatise; the first fascicle, which is in preparation, will contain an Introduction and descriptions of the Proteo- myxa, Mycetozoa, Lobosa, Heliozoa, Labyrinthulidea, Radio- laria, and Flagellata, forming sections A to G of Chapter I— The Protozoa. The division of the work into Chapters, of which the second to the twenty-first are already published, has resulted in a somewhat awkward restriction of the Protozoa to nominally vi PREFACE one chapter, the first. ‘This unduly large chapter is broken up into sections which serve instead of the usual division of so large a number of pages into chapters. The parts of the Treatise on Zoology dealing with the Mollusca, the Arthropoda, and the Vertebrata are in active preparation. E. RAY LANKESTER. May 15, 1903. CONTENTS CHAPTER I.—PROTOZOA. (continued) SECTION H.—THE STRUCTURE OF ANIMAL AND VEGETABLE CELLS . a I.—THE FORAMINIFERA = K.—THE SPOROZOA iy L.—THE INFUSORIA INDEX PAGE , a CHAPTER [I—PROTOZOA (continued) SECTION H.—THE STRUCTURE OF ANIMAL AND VEGETABLE CELLS ! IN reviewing the course of development of our knowledge of organic nature, there stands out one epoch-making discovery, that of the chambered structure of plants, made by Hooke in 1665, which was destined not only to profoundly modify the older con- ceptions as to the intimate organisation of animals and plants, but also to place in clear relief the fundamental unity which underlies Facsimile of part of a figure by Hooke representing cells of vegetable tissues (cork). the apparently endless variety of external form. But, as in the case of most discoveries of wide-reaching import, the general recognition of the true nature of the cell did not emerge at once in its modern form, nor was it in reality the outcome of the work of any single investigator. Indeed, nearly two hundred years elapsed before the first enunciation of the doctrine of a cellular structure of plants 1 By J. B. Farmer, D.Sc., M.A., F.R.S. (1902). I 2 THE STRUCTURE OF CELES by Hooke became translated into a form comparable to that in which the phrase is now understood. Nevertheless to Hooke belongs the credit of having not only depicted the vesicular nature of cork and other plant-structures, but also of having designated the cavities by the name of cells. Malpighi and Grew in the succeeding century had fully recog- nised the cellular character of plants, and even attempted a crude explanation of the origin of the cells themselves, likening them to the vesicular foam of beer. But accurate as was their portrayal of the mature structure, they nevertheless possessed no real concep- tion of the true meaning of the cell as the unit of organic life. The cells were regarded as the cavities in the matrix, not as the units which together constitute the organism, and it was to the wall that all their observations were directed. Little or no attention was seriously paid to the cell-contents. Thus, although Corti in 1772 had noticed the rotation of the viscous matter in the cells of Chara, his discovery remained without influence, and was made again, and independently, by Treviranus some forty years later. Even the discovery ! of the nucleus by R. Brown in 1831-33 failed at once to excite the interest of the majority of his contemporaries, nor indeed does it appear that Brown himself at all fully grasped the signifi- cance of his discovery. Whilst in the plant-body it was the cellular structure, in the sense of Hooke, Malpighi, and Grew, which most forcibly appealed to the observer, the softer tissues composing an animal body were not so easily referable to a similar plan, although a consideration of the blood corpuscles, and of cartilage, helped to pave the way for the later generalisation. But, on the other hand, the animal body was more suited to turn the attention of the investigator upon the living substance, and the fundamental importance of the latter seems to have been first clearly appre- hended by Dujardin, who (in 1835), gave the name of Sarcode to the contractile, gelatinous, diaphanous mass constituting the bodies of the Infusoria which he was examining. He even succeeded in distinguishing some structural details, but with the lens at his command it may perhaps be doubted whether this really repre- sented more than the arrangement of granular and other inclusions in the living substance. It was, however, chiefly due to the researches of Schleiden, and especially of Schwann, which were published in 1837-38, that general interest became steadily focussed upon the cell-contents, including the nucleus which formed a cardinal point in their famous cell-theory. And it is largely to the great influence exerted by their work that the rapid advances witnessed during the next succeeding decades are legitimately to be traced. It is, of course, 1 Others, including Leewenhoeck, had already seen nuclei in isolated cases, but their observations were quite without influence on the development of thought. THE STRUCTURE, OF. CELLS 3 true that the greater part of their conclusions, especially such as are related to the genesis and growth of cells, have since turned out to have been erroneous. ‘This is largely due, perhaps, to the weight of mistaken preconceptions on their part ; but the history of advance in any line of thought or science is full of similar examples. It is sufficient that they realised to the full the immense importance of the inquiry, and at any rate they succeeded in correlating and in co-ordinating a large mass of observations, and so became the means of immediately attracting numerous other workers into the same field. ‘ To Schwann may be conceded the merit of having first con- sciously attempted to demonstrate, in the most effective manner, the essentially similar character of the cells in plants and animals. This he did by endeavouring to follow out the origin and develop- ment of new cells in each of the two great divisions of living organisms, though how wide he was of the practical truth may be seen from the account which he gives of the process. The primordial substance out of which cells are formed consists, according to him, of a gelatinous or slimy mother-liquor, the cytoblastema. In this, by a process of condensation, a nucleolus is first formed. This then grows by intussusception, and gives rise to a nucleus, in which once more a nucleolus is differentiated— itself the origin of another nucleus. Meantime, from the cyto- blastema fresh matter is deposited in the surface of the nucleus, and thus a consolidated membrane originates. This membrane, by intercalation of constantly increasing material within it, continues to grow, and ultimately it forms the wall of the new cell, the contents of which are provided for in the way just described. Thus, in the formation of cells, according to Schwann, the following stages, starting from the raw material—cytoblastema—may be distinguished. First, the condensation giving rise to the nucleolus, this in turn, by growth, produces the nucleus, and the peripheral (nuclear) wall eventually forms the wall of the new cell. At first sight it is difficult to realise how these ideas obtained the wide currency which they enjoyed, but the reason is to be sought in the fact that Schwann, like Schleiden and Nigeli after him, was not fortunate in the material he selected for investigation. Cartilage, blood-corpuscles, and pollen grains were repeatedly studied, and it is perhaps not surprising that with such objects before them an incorrect conclusion was arrived at. Von Mohl, who had been engaged in studying the structure and mode of division of vegetable cells since 1835, at one time gave a true explanation of the process, but afterwards he sounded a less certain note, adhering to the view that the new cells were formed in toto within the mother cells, even in the case of algal filaments— an error which was definitely opposed by Unger. Von Mohl 4 THE STRUCTORE-OF CELLS clearly recognised the importance of the formative substance of the cell, to which in 1846 he gave the name it now bears, viz. Protoplasm, the same word that had already been employed six years earlier by Piirkinje to designate the formative substance met with in the animal embryo. Speculation was already aroused as to the possibility of instituting a comparison between vegetable protoplasm and animal sarcode, and Dutrochet had, as early as 1824, and still more definitely in 1837, indicated the general similarity which underlies the structure of animals and plants. But it was reserved for Cohn to clearly formulate in 1853 the real features of identity between them, and to express the definitely reasoned view that they were, in all essentials, composed of the same kind of substance. Cohn was well fitted for the task, by his acquaintance with the lower organisms of both animal and vegetable kingdoms. Max Schultze in 1861 further elaborated this resemblance, and his convincing demonstration at once gained the assent of all who were competent to form an opinion on the question. Moreover, Schultze clearly saw that it is the protoplasm (in the widest sense of the term) which essentially constitutes a cell, and he, like Leydig, defined it as a mass of protoplasm containing a nucleus. About the same time also Virchow, in his celebrated aphorism, ‘‘ Omnis cellula e cellula,” crystallised the correct view as to the general mode of origin of new cells. But although the essential facts of cell-structure and development thus gradually emerged from the earlier and cruder notions, the finer details, and especially the relations of the nucleus, long remained obscure. The origin of this body, and its connec- tion with the rest of the cell-contents, was not understood, and a very general view was held that it disappeared (as indeed is in a certain sense correct) at each cell-division, to be formed afresh in the new daughter cells. It is true that so long ago as 1841 Remak had put forward the statement that the nucleolus and nucleus gave rise by direct fission to the corresponding structures in the daughter cells, and indeed that the whole process of cell-division was thus inaugurated; but his views (which for a few cases are really well founded) appeared to be not generally applicable, and thus it transpired that even in the middle of the century the nucleus came to be commonly regarded as an organ of but secondary importance, and this even by so eminent an investigator as Briicke. It was not until the publication of Strasburger’s mag- nificent work on the cell- and nuclear-division in 1875 that the nucleus received its proper share of attention. Strasburger, some four years later, like Virchow, in another connection, before him, defined the modern position in the phrase ‘‘Omnis nucleus e nucleo.” The researches of the brothers Hertwig, Van Beneden, Flemming, DAE STRUCTURE.OF CELLS 5 and others have abundantly emphasised and justified these far- reaching generalisations. But with the improvements which have been effected in technique during the last quarter of a century, new facts have come to light which have somewhat modified the conception of the cell as held by the earlier writers. It has been already seen how the centre of gravity gradually shifted from the cell-wall to the cell-contents, and that, as Max Schultze declared, the essential constituents of the cell were represented by the protoplasm and the nucleus, the wall being of altogether subordinate importance. The discovery that masses of protoplasm might contain not one but many nuclei, and that such a condition is not uncommon both amongst various groups and tissues in plants and animals, appeared to some writers to present a difficulty in accepting the cell theory as treated above, and various explanations have been offered in order to bring these cases into line with the theory as more generally understood and defined. Such organisms or tissues have been termed non-cellular—a negative and unsatisfactory expression which has been replaced by the more appropriate word syncytium or coenocyte. These words emphasise the view that, mor- phologically, the individual units, which collectively make up the syncytial tissue, are not isolated from each other by definite barriers. Sachs proposed the term energid to express the cell in Max Schultze’s sense, meaning thereby the nucleus, together with the portion of protoplasm dominated by it. Essentially this is a physiological definition as contrasted with the morphological idea embodied in the word syncytium. And it is, on the whole, a legiti- mate expression, since it really does correspond to a fact. Moreover, it has the merit of being equally applicable to the cases of isolated cells as well as of those in which such limits are not structurally traceable. The objection raised to the conception underlying it, on the ground that the nucleus of a syncytium does not always dominate the same protoplasm, is not a valid one, inasmuch as it is quite possible—perhaps even probable—that an essentially similar state of things also obtains even in those tissues in which the constituent cells are apparently isolated. For it has gradually been proved for a very large number of cases that the protoplasm of adjacent cells is in actual physical continuity through the fine pores present in the delimiting cell-walls. It is tolerably easy to observe this continuity in the epithelial cells of some amphibian tissues, and there is a con- siderable weight of evidence to show that it is far more general than was generally supposed to be the case. In plant-tissues also it has been repeatedly demonstrated since its first discovery by Tangl (in 1879)in the endospermof certain seeds. Gardiner and Russow almost simultaneously demonstrated its existence in the tissues of several adult plants in 1883, and since that time it has been clearly proved to 6 THE STRUCTORE OF. CHLEES occur throughout the tissues of the organism in those examples which have been specially investigated for the purpose. Thus it is evident that, so far from the syncytial condition presenting an exceptional case, it is in reality an extremely common one, the cell-walls merely forming a perforated skeletal framework which supports the softer parts. It is useless to argue (as has recently been done) that the pores are so fine as to be practically useless for the transference of material substances through them, since, as Horace Brown has shown, their very narrowness, taken together with the thinness of the portion of membrane on which they occur, is an important condition for the performance of such a function without unduly weakening the framework itself. Moreover, the existence of the continuity of the protoplasm from cavity to cavity at once renders intelligible the Fic. 2. Continuity of protoplasm through vege- table cell-walls. A, cells of the pulvinus of Robinia. B, cells of the endosperm of Heterospathe. (After Gardiner.) possibility of a transmission of stimuli from one part of the body to another, although it would perhaps be going too far to assume this as a necessary condition of transmission. Examples are known in which the stimuli appear to be less directly conveyed, as, for instance, in the case of nerve ganglia, according to Ramon y Cajal (although his results have not been upheld by some other investi- gators); and further, in some plant tissues, water squeezed out into the intercellular spaces has been regarded (on rather slender grounds) as being the means of exciting consecutive cells of a tissue. On the other hand, Némec has recently shown reason for admitting, in the irritable parts of plants, the existence of specialised tracts of protoplasm which are continuous from cell to cell, and by the agency of which the stimulant impulse is conveyed to the motor executive region. Whilst the general and mutual relations of the constituent parts of the cells were being gradually elucidated, it became recog- nised that the cellsubstances themselves were not composed of THE STRUCTURE OF CELLS - mere structureless jelly, but possessed an organisation of their own. At first the recognition of this fact only appears in tentative sug- gestions, and hardly any serious progress was made beyond the obvious distinction of the nucleus from the protoplasm. Briicke seems to have been the first to point out the philosophical necessity of assuming an organisation in the protoplasm, but the visual per- ception of the counterpart of such a constitution hardly advanced beyond the recognition of a relatively solid mass bathed in a more fluid substance. The former was distinguished as spongioplasm, and the latter as hyaloplasm. It is significant of the difficulty experienced in arriving at a definite decision on the then available evidence that each of the two constituents has been claimed by different writers as the living substance. The views which have been put forward as to the relationships of the various substances which co-exist in the protoplasm to each other have developed in two principal directions. The earlier, his- torically speaking, was advocated by Frommann in a series of papers dating from 1864. He was led, by a study of nerves, to distinguish a reticulum, which partly corresponds with Leydig’s spongioplasm. This reticulum was imbedded in a more homogeneous ground sub- stance, which, however, includes much more than spongioplasm. He extended this conception of protoplasmic structure to plant cells, and it was utilised, and in some respects modified, by Heitzmann in 1873. The views of the latter author were not so convincing as those of Frommann, for it is quite possible to identify the structures described by the latter writer in living cells, although the appear- ances are susceptible of a different interpretation from that given by him. Heitzmann’s descriptions, on the other hand, are very schematic, and it is difficult to avoid the conviction that they are highly coloured by theoretical preconceptions. The phenomena of contraction and extension were brought by him in relation to the structures as described, but his views have never met with very general acceptance. A closely related hypothesis was that suggested by Flemming, who, while denying the existence of a reticulum, insisted on the presence of a fibrillar structure, the fibrils being represented as threads of irregular length (the filar elements) which were imbedded in a more fluid interfilar mass. Gradually another view of the structure of protoplasm was evolved, and which, in a measure, took account of reticular structure, but explained it differently. Strasburger in 1876 first seems to have spoken of closed protoplasmic chambers, which were filled with more fluid albumen, but he soon abandoned the idea in favour of the reticular hypothesis. But the alveolar theory thus indicated was developed and extended by Biitschli, who had, as long ago as 1873, figured in Pilidium a structure susceptible of such an explanation, though this was not given at that time. The alveolar 8 THE STRUCTUREOF, CELES theory has exerted considerable influence upon contemporary thought, and may be given in brief outline. The whole protoplasm, including the nucleus, is conceived of as in a physical condition resembling an emulsion, the more fluid mass filling the cavities (which are very small, 1-2 » in diameter), whilst the walls are composed of a more viscous substance. Such an emulsion obeys certain well-known physical laws, and the relation of the alveolar walls both to each other and to the free outer surface can be theoretically defined. Solid heterogeneous particles enclosed in the emulsion, if too large to lie in the substance of the walls, are surrounded by a surface film in which the alveoli are arranged, as they also are on the free surface, somewhat like the cells of columnar epithelium. Move- ments may occur in the whole mass, as the result of disturbances in the surface tension of the superficial alveoli, and movements pro- duced in emulsions in this way closely resemble the streaming and other movements of protoplasm. The reticular Vis and filar appearances are also to be attributed ; : to disturbances, due to causes, in the interrelation of the alveoli, or they may represent the optical section of the alveolar walls themselves. Now it is quite possible to convince oneself that such an alveolar structure does actually exist in many cases, although, as Biitschli himself admits, it is not always to be so identified. It seems probable, however, that on the whole it does represent truly the appearance of protoplasm under certain (and commonly occurring) condi- tions, but it also seems equally clear that these conditions are not necessarily always fulfilled. For it is essential for the production of such an appearance that there shall be at least two non- miscible fluids of different refractive index, and if either of these conditions is not realised, or is temporarily in abeyance, it will follow that the Fic. 3. alveolar appearance must also be absent or dis- The foam structure appear. And we are acquainted with so many of the protoplasm ofa . : j . gregarine. important series of changes in the relations of the various protoplasmic constituents to each other that it is hardly necessary to postulate the permanence of those conditions of which an alveolar structure is the consequence. Thus it would seem that an easy modus vivendi might be reached which would render it possible, whilst recognising the heterogeneity of the substances included under the generic term Protoplasm, to admit that at one time an alveolar, at another a filar, or a reticular appearance might occur. A fibrillar structure is certainly present during nuclear division, and although the extreme adherents of the THE STRUCTURE OF CELLS 9 alveolar theory see in the fibrils a honeycomb structure, the cavities are generally admitted to be reduced to the vanishing point. Strasburger has attempted to utilise both the filar and the alveolar hypothesis, considering that in every cell the protoplasm outside the nucleus consists of two distinct parts. The one, which is specially nutritive and alveolar, he terms Zrophoplasm ; the other, which is more closely concerned with the dynamical changes in the cell, and possesses a filar structure, he designates as Kinoplasm. 'The relations of the kinoplasm will be more specially considered in connection with nuclear division. Besides the protoplasm and nucleus, there are present other organised structures in the cell. The vacuoles, which have long been recognised, are cavities in the protoplasm, and lined apparently with a specialised layer of this substance. In some cases they are rhythmically contractile as in many of the Protozoa. But it is around that enigmatical body, the centrosome, that especial interest has persistently attached ever since its first definite discovery by Van Beneden in 1885. The centrosomes are minute granules, most often situated either singly or in pairs in each cell, and in close proximity to the nucleus. They are frequently con- tained in a specialised mass of protoplasm, termed by Boveri the Archoplasm. Centrosomes and their attendant structures have been differ- ently described by various observers. Van Beneden, to whom we owe the first recognition of these bodies, distinguished, in the case of Ascaris, a central granule, surrounded by two concentric areas, to which he gave the names of medullary and cortical zones respectively. Boveri described in the cells of the same animal a centrosome surrounded by a lighter zone, from which it was definitely eut off by a kind of limiting membrane. Within the centrosome he further dis- tinguished a central granule, the cen- triole. The latter body divides before the centrosome increases by fission. In still other cases (¢.g. in cells of the testis of Salamander) various observers (Meves, Driiner, etc.) have distinguished Fic. 4. a whole series of concentric zones Ascaris megalocephala. Schematic around the centrosome. In the giant Ciavage mitosis. centrosomes ma cells of the spinal cord and in leuco- Qiu ven Baden CCM 70R° eytes, Heidenhain has distinguished a group of granules, which replace the single or paired one more commonly met with, and in other cases again there is a reticulate sphere (echinoderms) containing a varying number of fo) THE STRUCTURE OF GELES granular inclusions. In plant cells centrosomes have been far less often identified than in animals. They are more frequent, or at least more easily demonstrable, in the lower members of the vegetable kingdom than in the higher plants, in which they are probably restricted to the motile sperms. The evidence for their occurrence in angiosperms is not convincing. When present in a cell, they usually occur in the form of a small granule enclosed in a sphere, and are comparable with the centrosome and centriole of Boveri. Strasburger has proposed the convenient term of centro- sphere to designate the sphere together with its included granule, reserving the term centrosome for the latter body only. It is quite certain that the centrosphere apparatus presents itself in varied degrees of complexity, not only in different organisms, but even in different cells of the same tissues, and Strasburger’s term has much to recommend it, since, in spite of the large litera- ture which has grown up around the subject, we are still mainly in the dark as to the true meaning and relations of the different parts. It seems clear, for example, that the centriole of Boveri corresponds to that which by most writers has been called the centrosome, and Boveri himself states that the division of his ‘ centrosome” is preceded by that of the centriole. The centrosome or centrosphere is itself not unfrequently enclosed in a denser mass of protoplasm, called by Boveri the Archoplasm, and by Strasburger. the Kinoplasm. Probably, how- ever, appearances denoted by these terms are not the expressions of permanent structures, but represent transient phases of cellular activity. The structures thus called into existence may, however, be, at least temporarily, very pronounced, since at least a part of the achromatic figure, which is formed during nuclear division, owes its origin to the archoplasmic mass. Nevertheless, the archo- plasm (or kinoplasm) may become absolutely indistinguishable at other periods in the life of the cell. A far more difficult question to answer than that concerned with the permanence of the archoplasmic or kinoplasmic structures refers to the centrosome itself in a similar connection. Whilst many authors have strenuously maintained its permanence from one cell-generation to another, comparing it in this respect with the nucleus itself, a considerable weight of negative evidence has never- theless accumulated in the opposite scale. The striking relations which obtain between the centrosome and the nuclear figures at phases of division naturally produce a profound subjective impres- sion upon the observer, and it has even been assumed that the centrosome still persists, even when its actual existence cannot be successfully demonstrated. There is no doubt that other granules have frequently been mistaken for centrosomes, selected because they happened to lie sufficiently near the spot where the structures DHE STRUCTURE OF CELLS II in question might have been looked for, and thus no little confusion has been introduced into a subject already sufficiently bristling with difficulty. But the cases of Acanthocystis (Schaudinn) and of Actinosphaerium (R. Hertwig) show quite clearly that centro- somes may, at least in the lower animals, be certainly differentiated afresh in the cells from which they had previously been absent. Fic. 5. aa Oe X PF ISS Shh © may My \: Acanthocystis aculeata. A and B, for- Oia mation of the centrosome from nuclear ix “ constituents in swarm-spores. C, resting \ ogee cell. D, nuclear division preceding fission. SGA" ree) eee (After Schaudinn.) ANE ae The so-called Blepharoplast, which is associated with the male reproductive cells of certain cycads and ferns, appears to present very strong analogies with animal centrosomes, and yet the blepharoplasts have not been seen in the antecedent cell-genera- tions of the plants in which they occur, and hence they have almost certainly been formed de novo. On the whole, then, the question as to the relative permanence of the centrosomes through the series of ontogenetic cell-generations must be left an open one. Certain facts are, however, known which conclusively prove that centrosome-like structures can be formed in cells from which, under 12 THE. STRUCTURE OF CELLS normal circumstances, they are absent. Morgan showed that concentrated solutions of salts could induce the appearance of centrospheres with radiations in the eggs of certain echinoderms, and Loeb further proved that by adding magnesium chloride in appropriate quantity to sea-water, the eggs of sea-urchins could be brought into such a state that when replaced in normal salt water Fic. 6. Actinosphaerium eichornii. A and B, nuclear origin of the centrosome, which arises at one end of the nucleus. C, D, further stages in the mitosis (A-D refer to the first polar mitosis). E, diaster of a somatic mitosis, in which no centrosome is present. (After R. Hertwig.) they underwent the normal embryonic segmentation. Wilson, investigating the cytology of the process, confirmed the results, and ascertained that the treatment caused the formation of centro- spheres which seemed to direct the cell-divisions. And R. Hertwig long ago showed that at least the early stages of a parthenogenetic segmentation could be similarly induced by the action of strychnine. Hence there is a considerable body of evidence to show that the centrosomes are structures which, though physiologically the signs oS ial iit SEROUCLO RI OF. CHEES 13 of important changes in the protoplasm, are not necessarily permanent organs of the cell. And a wide survey of the processes of mitosis in the lower animals and plants serves fully to confirm this conclusion. The Structure of the Resting Nucleus. It has already been said that it was not until the year 1875 that the nucleus was fully and universally recognised as an all- important cell organ. Even as late as the previous year, Auerbach published a treatise on its behaviour during cell-division maintaining that it completely disappeared during the process, and he gave the name of Karyolysis to the phenomenon in question. With the recognition of the complex series of changes undergone by the nucleus during division, and its obvious importance, in connection with fertilisation, also discovered in 1875, it speedily formed an object of serious study. And the investigations were not only carried on in killed cells, but its behaviour during life, as well as its chemical structure, presented attractive problems for solution. The general outcome of these investigations is as follows :—The nucleus is delimited from the cell protoplasm (the cytoplasm of Van Bambeke) by a membrane which was regarded by Schwartz as consisting of a substance called by him Amphipyrenin. In some cases, however, it appears not improbable that the membrane is at least partly produced from the cytoplasm, as a kind of precipitation membrane, whilst in other cases, as for example in some of the coccidia, Schaudinn has shown grounds for thinking that the so-called chromatin of the nucleus itself may contribute to its formation. Within the membrane the nuclear contents may be distinguished as a matrix of a substance which stains with some difficulty, and which forms a sort of meshwork within it. This is the Linin of Schwartz, and seems to closely correspond with the plastin, distinguished chemically by Zacharias. In addition to the linin there exists a more fluid gelatinous substance, the Paralinin of Schwartz. Imbedded in the linin are a large number of granules which, by reason of their exhibiting a strong affinity for certain dyes, were termed Chromatin by Flemming. The chromatin consists of a highly complex nitrogenous substance, and always contains phosphorus. Chemically it belongs to the class of proteid compounds classed as nucleins, and by analysis can be made to yield proteids and nucleic acid. In addition to the true nuclein chromatin, there have been described other inclusions within the linin known as Lanthanin (Heidenhain) or oxy- or basi-chromatin bodies, which appear to be related to the nuclein series, and which perhaps are complex, high-graded substances which can be built still further up to true nucleins. Most nuclei contain, besides these constituents, one or more 14 THE STRUCLURE OF CELLS masses, usually of a spherical or oval shape, known as Nucleoli. These bodies long ago attracted the attention of investigators, and it will be remembered that they were raised to a rank of con- siderable importance by both Schwann and Remak. They usually are easily stained, and thus were included amongst the chromatin bodies of the nucleus, but subsequent investigation has shown that they are, in many cases, widely different from nuclein. Two kinds of nucleoli were distinguished by Flemming under the names of eu- and pseudo-nucleoli respectively, the latter representing, at least chiefly, aggregations of a substance which closely approximates to, if it is not actually identical with, true nuclein. And more recent investigations have tended to confirm the supposition advocated by Zacharias, that the ordinary eu-nucleoli, so far from consisting of a single substance such as pyrenin (Schwartz), are complex mixtures, or else, at any rate, bodies which readily yield, by suitable treat- ment, different substances of complex molecular composition. It is true that the author just referred to arrived at the conclusion that the nucleoli were destitute of phosphorus, but this view can hardly be maintained, at least in all cases. Investigations on the nuclei of Protozoa and of some of the lower plants seem to have shown that these nucleoli consist of at least two groups of substances, the one consisting of, or approximating to, nuclein, the other more nearly resembling the linin, or even the cytoplasm, in its staining and other reactions. At any rate, the chromatin, which forms so obvious a character in dividing nuclei, appears in some cases, ¢.g. Actinosphaerium, to be mainly derived from a nucleolar source. It is highly probable that these bodies are really heterogeneous, and represent reserves of complex materials which can be drawn on for various purposes during periods of nuclear activity. For at such times the nucleolus always undergoes considerable change, and is either completely used up, or its remains fragment and pass out into the cytoplasm, where their further fate is still obscure. Whilst the nucleoli are thus losing substance they often exhibit vacuolation, and even in resting nuclei vacuoles may sometimes be detected within the nucleoli, pointing strongly to the correctness of the hypothesis as to their heterogeneous nature. A further point which deserves mention in connection with the nucleoli is the view held by some writers (e.g. R. Hertwig) that they stand in some close relation to the centrosomes, and that the existence of the latter structures may be traced back to a nucleolar origin. Further, Strasburger, in his account of kinoplasm, has suggested that the nucleolar substance may serve as the material which stirs up the dynamical and metabolic activities latent in the cell. On the whole, it is impossible, in the present state of our knowledge, to ascribe any single function to these bodies, and the THE STRUCTURE OF CELLS 15 evidence before us seems to indicate that just as they are very diverse in structure and composition, so also they may, and almost certainly do, play very different parts in the general economy of the cytoplasm and nucleus. The resting nucleus may, then, be regarded as an organised structure containing a considerable assortment of highly complex and labile substances. But this very lability, itself a condition of the profound and important changes which succeed each other with extraordinary rapidity during the division of a nucleus, is bound up in, or at least is related to, an organisation which directs and Attraction-sphere enclosing two centrosomes, Plastids lying in the cytoplasm. true nucleolus, —_{- Plasmosome or Chromatin- network. < Nucleus Karyosome or net-knot. Vacuole. Lifeless bodies (meta- plasm) suspended in the cytoplasmic reticu- lum. Fic. 7. Diagrammatic representation of the structures present in a typical cell, (After Wilson.) determines that sequence of chemical and physical transformations which so strikingly accompany the whole process. Moreover, there is abundant evidence of the existence of a material exchange passing between the nucleus and the cytoplasm which becomes strongly marked at all periods of special cellular activity—such, for example, as secretion, regeneration, and the like. Nuclear and Cell Division. The multiplication of the uninucleate cell is always preceded, save in the lowest protozoa and protophyta, in which the details of the processes are still obscure owing to the absence from them of 16 THE. STRUCTURE OF CELTS a well-defined nuclear body, by a division of the nucleus. This may either take place in a simple manner, as was determined by Remak, or it may only be secured as the result of a complicated rearrangement and fission of certain nuclear constituents. To the former, or direct (Flemming), method of division, the term Amitosis has been applied by Flemming, whilst the latter, or indirect (Flemming), method was also termed Mitosis by the same author. The word Karyokinesis (Schleicher) has often been substituted for mitosis, but both terms are expressive of the same phenomena. Amitosis, in the higher animals, is not of such generally widespread occurrence, but in the lower forms it frequently appears as an intercalated method along with a more or less complex form of ordinary karyokinesis. Itis also generally met with in nutritive gland cells; thus in the follicular epithelium of the ovary, in the ‘‘foot ” cells of the testis, and in the tapetal cells of the higher plants, it is not uncom- mon. In these cases it appears to be characteristic of degenerating tissues, and this explanation has been extended to amitosis gener- ally by Ziegler and vom Rath, but many instances are known in which such a view is quite untenable. Thus, there is good reason to believe, as Meves or others have shown, that in the ovary, cells which ultimately are destined to give rise to ova may multiply in this way, and Schaudinn, Siedlecki, and others have shown that in the Sporozoa such amitotic divisions often follow shortly on the act of fertilisation, and give rise directly to the new generation of parasites, and again amongst Infusoria the macronucleus seems always to in- crease in this way. Furthermore, Nathansohn proved, in the case of Spirogyra, that by appropriate treatment with anaesthetics the nuclei could be induced to divide amitotically, and that this amitotic origin in no way influenced the conditions of the subsequent develop- ment of the cells concerned, for these were capable of even pro- ceeding as far as to form sexual cells, on the restoration of a normal environment. But in comparing amitosis and mitosis together, the advantage which the latter possesses, so far as can at present be stated, seems to lie in the accurate quantivalent distribution of all the structural elements concerned in the process between the two daughter nuclei. Whether this is the only advantage, or whether perhaps some mechanical or other factors are also involved, must be left to the future to decide. In considering the general phenomena presented by karyokinesis, there are two sets of factors which, though closely interwoven in the process, may with advantage be kept as distinct as possible. For these changes involve the nucleus on the one hand and the cytoplasm on the other, and the degree of complexity which each of them may respectively assume is not necessarily invariable or correlative, either in different organisms or in different tissues of the same individual. A second, and not less important, considera- Ee STRUCTURES OF CELLS 17 tion arises in connection with the fact that nuclear- and cell- (or cytoplasmic-) division are by no means invariably associated, and that although the cytoplasm never gives rise to a number of cells in excess of the number of nuclei present, its divisions in other respects may occur independently of that of the nuclei. This is seen in the cleavage of some animal eggs (¢.g. mollusca), in the formation of endosperm in the seeds of angiosperms, in the develop- Fic. §S. Liliwm martagon, prophase of the first mitosis in the pollen-mother-cell, showing the longi- tudinal fission of the cromatin and linin. ment of the eggs of Fucus and of the spores of Mucor, In all these instances, the division of the nucleus precedes that of the cyto- plasm, which is only subsequently partitioned. The first indication of approaching karyokinesis in an ordinary somatic cell of the body of one of the higher plants or animals is usually visible in the nucleus. The chromatin granules become ageregated in lines, corresponding to a growing definiteness in the delimitation of the linin. Thus from the generally granular appear- ance, the character of a much convoluted and tangled chromatic 2 18 THE STRUCTURE OF CEEES skein is produced. ‘The linin framework does not necessarily form a continuous thread. Often it is more or less broken, and it almost always shows cross-anastomoses (from which, however, in the later phases, the chromatin is commonly absent) with the neighbouring threads. ‘This anastomosis is doubtless the expression of its segrega- tion, due to contraction ; the anastomoses themselves representing the original meshes by which the substance was formerly bound together into a coherent whole. Simultaneously the chromatin increases largely both in amount and in the intensity of its stain- ing power—a fact which may be taken to indicate a chemical or physical change in its state. The linin thread-work that contains the chromatin is often not scattered irregularly through the nucleus, but is more or less polarised, as was clearly observed by Rabl, in such a way as to converge, often with considerable distinctness, towards one point on the nucleus. This point is occupied by the centrosomes when they are present. At first usually lying in pairs, and often in a mass of archoplasm, these bodies in the simpler cases now commence to diverge, and each is either accompanied by a portion of the original archoplasm, or else the latter is differenti- ated progressively and afresh as they move apart to take up diametrically opposite positions on the periphery of the nucleus. From them radiate outwards into the protoplasm the well-known astral figures which are characteristic structures in the cell at this period, and are commonly regarded as of archoplasmic origin. Meantime within the nucleus the chromatic thread thickens and shortens. Some of its substance is probably derived, at least in many cases, from the nucleolus, which becomes vacuolated and often fragments about this stage. Finally, the thread breaks up into a number of segments which is constant for the somatic cells of the species. These segments are the Chromosomes (Waldeyer). At or immediately following this stage a fibrillar structure begins to appear within the nucleus, and as it increases the chromosomes are gradually driven to occupy an equatorial position (the equatorial plate stage) in the nucleus. What is precisely to be looked on as the origin of these fibrils (the so-called achromatic fibres which together form the achromatic spindle) is not certainly known. Some, with Strasburger, hold that they are exclusively of cytoplasmic (kinoplasmic) origin, growing inwards, as it were, from the polar centrospheres. Others again look on them as derived from nuclear substance, whilst a third view regards them as of mixed origin. Probably the last view is less open to objection than the other two. The fibrillar structures themselves are almost certainly the result of conditions of stress and strain in the viscous substances of which the cell is composed, and it would appear probable that any sub- stance capable of assuming the fibrous character might be compelled to do so. And there is abundant evidence to show that such sub- RAE SERUCTORE OF CEELS 19 stances do exist in the nucleus, as they certainly do in the cyto- plasm. For in the case of, e.g., the micronuclei of infusoria, the whole spindle is entirely intra-nuclear, the cytoplasm apparently not furnishing, at least directly, any part of it. With the congregation of the chromosomes to form the equatorial plate, the first stage or Prophase of division terminates. The equatorial plate, or aster, stage is often one of relatively long duration ; so much so that it may even happen that some of the signs of cytoplasmic activity may fall into temporary abeyance. For example, the astral radiations outwards from the centrosomes may cease to be visible at this stage (¢.g. in Pellia), though they Fic. 9. Stages of the mitosis in the micronucleus of Paramoecium, showing the “pole plates”; true centrosomes are not present. (After R. Hertwig.) reappear later on. The nuclear wall commonly, though not always, disappears whilst the chromosomes are collecting at the equator, and the nucleolus or its fragments, if they have not previously disintegrated, now are no longer recognisable. The individual chromosomes often, but by no means always, assume the shape of a V, with the apex turned centrally in the equatorial plane. Each one is supported by fibres of the achromatic spindle which run from the poles, and terminate on the chromosomes at the equator. The chromosomes next split longitudinally, and this partition forms the commencement of the stage known as the Metaphase. The two daughter halves rapidly diverge, being guided by the spindle fibres towards the poles. During their divergence 20 THE STRUCTURE OF CGELES Fic. 10. Fucus vesiculosus, stages in the first mitosis in the fertilised egg (odspore). A-D, Prophase ; £, commencement of the Metaphase. EAP STRUCTURE OF CHEEES 21 Ae ‘ ' % A RDN, Hh 9 he aN, Pd te +O % fe Fic. 10 (continued). Fucus vesiculosus, stages in the first mitosis in the fertilised egg (odspore). F, Metaphase ; G, H, Anaphase. THE STRUCTURE OF CELES to N fresh fibres are differentiated between the retreating groups of halved chromosomes, and form the interzonal fibres (Verbindungs- fiiden of the German writers). Whether these play any mechanical part in forcing the daughter chromosomes apart is uncertain, as is also the réle assigned to the above-mentioned fibres that appear to direct the chromosomes towards the poles. Probably the latter Fic. 10 (continued). Fucus vesiculosus, stages in the first mitosis in the fertilised egg (odspore). J, Telophase. (Phil. Trans. of the Royal Society.) ; are actively contractile, and there is some evidence to show that the interzonal fibres are in a state of stress. In some instances, ¢.g. in fertilised and segmenting eggs of Fucus, the arrangement of the elongated plastids in this region plainly indicate such a condition of stress or strain. But the achromatic apparatus varies considerably in the degree of its complexity, and it probably would be unsafe to attempt to assign constant functions to its constituent parts. So much, however, may be said, that the chromosomes appear to be pie STRUCTURE (OF CELLS 23 passively moved to their respective poles, and to possess no power of automatic movement of their own. With the arrival of the chromosomes at their respective poles the Anaphase stage supervenes. This consists practically in a series of regressive changes which leads to the formation of normal resting nuclei. The chromosomes lose their sharp outline and swell up; at the same time the nucleoli once more reappear. The chromatin, or as much of it as persists, is distributed through the swollen linin just as it originally existed in the parent nucleus, and finally a wall isolates the daughter nucleus from the surrounding cytoplasm. But the cytoplasm still bears traces of recent dis- Fie. 11. Pelvetia canaliculata, telophase ot the second mitosis in the fertilised egg (odspore). (Phil. Trans. of the Royal Society.) turbances, and the period of gradual restoration of quiescence in it forms what is sometimes known as the 7elophase. The centrosome (when present) is often already doubled during the meta- or ana-phase, but the astral radiations frequently do not die away till much later. It is in the region of the interzonal fibres that events of the greatest interest are now proceeding. In animal tissues it very often happens that the two cells are constricted equatorially, and they may ultimately become delimited from each other, the remains of the interzonal fibres then remaining at this spot, where they may be long recognised as the Intermediate Body. In plants, owing to the existence of a cellulose skeleton, and the close adherence of the cytoplasm to its internal surface, such a con- striction does not usually arise. Instead of this the fibres increase 24 THE STRUCTURE (OF CELES greatly in number, especially in the equatorial zone. In these fibres the primordium of the new cell-wall is laid down, in the first instance as a viscous film, but which later, by the deposition of fresh substances, becomes converted into the cellulose partition. Its mode of formation is interesting because reasons have been shown for supposing that some at least of the protoplasmic connections between adjacent cells are primarily effected by the permanence of such continuity through the membrane during its formation. Diagram of the successive stages of a nuclear division. A, spire, with the fission of the chromatic linin. B, aster. C, D, E, separation of daughter segments. F, reconstitution of daughter nuclei. (After Flemming.) It may, however, happen that no membranes are formed in the interzonal fibres, such as will serve to delimit the daughter cells from each other. A complete series can be traced between the two extremes. Thus in the first division of the spore-mother-cell of Fegatella (Fig. 13), a cell plate is laid down, but not completed, and it is not until after the next nuclear division that this wall (which has shifted its position in the interval) becomes part of the final partitioning membranes. Again, in the endosperm of seeds, some- times the embryosac is transversely divided after the first karyo- kinesis, but far more commonly a large number of nuclei are first LAE STRUCLORE (OF CHEES: 25 formed. ‘These then take up their final positions, and a new set of interzonal fibres are differentiated between them, and in the equatorial planes of these groups of fibres the cell-walls are laid down. And finally, in other cases the cytoplasm may divide into masses containing either single or several nuclei, and_ secrete membranes without the intervention of interzonal fibres at all. It will have become evident from the foregoing account of the relation between the nucleus and the cytoplasm, that these two principal constituents of a cell retain to a consider- able extent a separate individuality, at any rate in the higher forms. This separate nature only be- comes obscured at periods of division, but even here, as has been seen, the essential boundaries are retained through all the changes connected with fission and redistribution. Thus it is legitimate to regard the cell nucleus as an entity which does not arise de novo. ‘The nuclei of successive cell- generations are lineal descendants of an ances- tral nucleus, just as the cells of the present day owe their being to the multiplication of ante- cedent parent cells. Fic. 13. The nucleus, how- Fegatella conica, the division of the spore-mother-cell z into four cells, showing the change in position of the ever, does not stand first formed wall. alone amongst the cell constituents as only arising by multiplication by fission of pre- existing structures of a similar character. In the plant-cell the various plastids originate in a similar manner, and there is no more evidence to show that they can be differentiated afresh from the general cytoplasm, than that the latter, by spontaneous generation, can arise de novo from its elemental constituents. The same is true for the curious coloured plastids known as Zoochlorella in animals, which possibly represent species of algae imprisoned in the cells of their animal hosts, or perchance, though less probably perhaps, they may be regarded as more akin to the chlorophyll corpuscles of the 26 THE STROCTORE OPAGELES plant cell. The latter hypothesis would be difficult to sustain in the absence of a series of forms through which their evolution might be traced, whilst, on the other hand, the symbiotic relation- ship existing between fungi and algal cells in a lichen strongly supports the presumption that an analogous case is furnished by the Zoochlorella organism and its host. “ Reduction Divisions.” Few cytological discoveries have aroused more widespread interest than that of the periodical recurrence of the so-called “ Reduction Divisions,” which are intercalated at some point in the cell-generations intervening between two consecutive sexual unions. Each uniting gamete or sexual cell contains in its nucleus only half the number of the chromosomes that will be characteristic of the embryo resulting from their fusion, and will be retained throughout its cell-generations up to those which lead in their turn, more or less directly, to the production of spermatozoa and mature ova. This remarkable phenomenon has been observed in all the animals and plants which have been carefully studied, with the exception of the more lowly or primitive forms in which the nuclear history is but imperfectly understood. The phenomenon in question was first made known by the investigations of Van Beneden in 1883 and 1887 on Ascaris. The choice of this animal was in some respects perhaps not very fortunate, since it does not exhibit the process in a very typical, but rather in an extreme, form, and thus a certain amount of misapprehension prevailed at one time respecting it. Since that period, however, very numerous animals and plants have been studied, with the result that the phenomenon is proved to be of very general occurrence, though differing considerably in detail in the various organisms. At first, and perhaps naturally, the view was advanced that the reduced number was secured through the mere degeneration and consequent elimination of the superfluous chromosomes, but it gradu- ally became clear that the evidence was entirely opposed to such a simple explanation, and that, on the contrary, the reduction was only arrived at after an exceedingly complex rearrangement of the nuclear constituents. It would, however, be going too far, as will subsequently appear, to deny that any nuclear substance is lost: all that can be said is that it is certain that no chromosomes, as such, are normally eliminated. In attempting to trace the sequence of events, it must be borne in mind that the process is evidently one of the highest importance, seeing that it occurs alike in animals and in plants, and this importance is increased rather than lessened by the further PEE STRUCLORE OF CELES No Sy recognition of the fact that the reduction may occur at morpho- logically diverse stages in the life-history of the various organisms —a fact which clearly emphasises its profound physiological sig- nificance. But although there is no lack of hypotheses to explain it, no one as yet has given a satisfactory theory which will embrace the whole range of the phenomena concerned. In the higher organisms the process of reduction appears invariably to be closely bound up with two nuclear divisions which rapidly succeed each other, and are hence often spoken of as the Reduction Divisions. These differ in some important respects from those characteristic of other mitoses. They can only be con- sidered in outline here, and after premising the existence of a not inconsiderable diversity as to the details of the process in different organisms. In animals the mitoses in question only occur in direct relation to the formation of the sexual cells or gametes, but in plants it is more usual to find a greater or less number of cell- generations follow on the Reduction Divisions before the actual gametes are formed. Thus it becomes obvious that the formation of sexual cells is not a necessarily immediate consequence of the change in the nucleus. If the course of events be studied in an animal, it is seen that in the development of the spermatozoon and of the mature egg, a strictly comparable series of changes is passed through. Just as the spermatocyte gives rise, by two successive bipartitions, to four sperms, so the immature ovum, by means of two successive nuclear divisions, gives rise to four potentially fertilisable eggs, of which, however, three commonly degenerate and are known as the polar bodies. The nucleus of the spermatocyte, just as does that of the im- mature egg (which may be distinguished from the ripe egg by the name of oocyte), goes through a somewhat prolonged period of growth before entering upon the critical mitoses, As these two divisions are marked by certain peculiarities from those of the other cell-generations, it is convenient to designate them by special names. The first may be termed the Heterotype, the second the Homotype, mitosis, following the terminology introduced by Flemming. During a large part of the growth-period, leading directly to the heterotype division, the nucleus cannot be correctly described as resting, for the linin reticulum is plainly discernible, as also are the regularly arranged chromatin granules, which serve to render it distinct. In fact, this prolonged spireme is highly characteristic of the heterotype mitosis, as contrasted with those which have been previously gone through. It is during this stage that the fission of the chromatin granules occurs, as was first seen by Pfitzner in 1881. Each granule becomes drawn out into a 28 THE STRUCTURE OF CELLS dumb-bell-shaped body, and finally two rows of granules are seen to occupy the margins of the flattened linin riband. This stage is often (Salamander, Helix, Lilium, etc.) followed by a more or less complete longitudinal fission and separation of the linin riband, each half now containing, at least at first, a single row of chromatin granules. It is perhaps not improbable that a similar process also occurs during the corresponding stage of somatic << _— ee Fia. 14, Lilium martagon, prophase of the first mitosis in the pollen-mother-cell, showing the longi- tudinal fission of the chromatin and linin. mitoses, but the shortness of its duration in the latter renders the process difficult to observe. At or about this period a remarkable contraction of the chromatic linin filament occurs, and commonly the nucleolus is included in the tangle. To this stage the name of Synapsis (Moore) has been given, and it seems to represent an important step in the process, and one which is confined to this first (heterotype) reduction division. After the synaptic condition is over, the linin, which has been getting richer in chromatin, is usually seen to be shortening, and at the same time thickening, but iE STROUCELORE OF CELLS 29 it may happen that the subsequent events become much obscured, the filamentous arrangement of the linin and chromatin ceasing to be distinctly recognisable. Up to this period there is, on the whole, a general agreement as to the nature and sequence of events, but the subsequent changes have been very diversely described and interpreted in the case of different organisms. In the most favourable cases the parallel arrangements of the chromatin granules and of the split linin thread can be followed for some time during the shortening and thickening of the filaments. Brauer has described, for the spermatocytes of Ascaris, a second longitudinal fission in each chromatic filament, resulting in the production of four rows of chromatin granules from the single row originally present in the primitive thread. A similar event has been stated by some to occur in the corresponding division in the pollen-mother-cell of a lily. In the majority of instances, however, the chromatic linin is seen to contract and thicken, and all traces of the fission may become unrecognisable. Finally, the chromatin comes to be aggregated in definite parts of the band, the intervening portions being occupied by colourless linin. There are often, also, cross anastomoses of the same substance between neighbouring strands. The chromatic areas in question mark the position of the developing chromosomes (Fig. 15, 4), which gradually become more definitely isolated from each other. And they are seen to be present in half the number characteristic of all the preceding nuclear divisions in the organism. Once more each chromatic band exhibits a split ' along the whole or greater part of its length, and this marks the line along which, later on, the cleavage of the chromosomes in this (heterotype) mitosis will be effected. In many cases, as is especially well seen in amphibians (Fig. 15, B, C), the fission of the young chromosome is incomplete and the sides diverge, thus causing the whole to assume the form of a closed ring. In other instances, however, the fission is completed, and the two halves, lying in close juxtaposition, may exhibit a complex series of figures which demand much care in their elucidation.” 1 Tt is commonly assumed that this split represents the original longitudinal fission of the linin filament. It is not, however, proved beyond doubt that this is invariably the case. 2 These appearances have, however, been differently explained by some investi- gators ; thus some have seen in them evidence of an approximation of two entire somatic chromosomes, hence when the apparent halves separate to give rise to the chromatic part of the daughter nuclei, it would follow that what has really occurred is the distribution of half the original entire somatic chromosomes to the daughter nuclei. That is to say, the division might be regarded as qualitative as well as merely as quantitative. And it will be evident on reflection that the same result might be reached as the result of various analogous interpretations of the fore- going processes, especially when the difficulties of investigation that occur during the synaptic tangle are borne in mind 30 LHE STRUCTURE OF CELES But the evolution of the chromosome does not always follow along these lines. In a number of instances, exemplified by many arthropods (e.g. Cyclops), after the early chromatic fission has been Fig. 15. Salamander, Heterotype mitosis in the spermatocytes. (After Meves.) For explanation of the figures see the text. . passed through, and the number of the future chromosomes has been marked out, these bodies, it is true, may form rings (Gryllotalpa) or parallel rods (Cyclops), but the chromatin, instead of being tolerably equally distributed throughout the length of the —.. THE STRUCTURE OF CELLS 31 longitudinal halves, becomes specially ageregated at two spots in each. Thus are formed the so-called Zetrads, to which much im- portance has been attached in the theoretical interpretation of the whole process of reduction. For it is thus seen that in the above eases the tetrad may be regarded as having originated first by a longitudinal fission of the chromosome rudiment, and then by a transversely isolated aggregation of chromatin in each half. It may also happen that tetrads are formed in a manner less easy to follow out, as in Helix and in Arion, in which the separate filaments are difficult, if not impossible, to trace in the stages immediately preceding their formation ; the chromatin thus appear- ing, so to speak, to travel to and become aggregated at definite areas, and to assume in a somewhat irregular manner the ring form of tetrad, similar to that occurring in Gryllotalpa, as described and figured by vom Rath. Still another type of tetrad formation has been described by A D C Fic. 16. Diagram illustrating tetrad formation. A, the split thread (spirem) stage. B, later stage, showing aggregation of chromatin at each end of the split bivalent chromosomes. 0, fully formed tetrads, of which the one to the right represents the most typical form. Brauer as occurring in the spermatogenesis of Ascaris megalocephala already alluded to above. The lining filament first contains a single row of chromatin granules, each of the latter divides crosswise into four, which lie in the same transverse plane, and hence the original filament now contains four rows of chromatin granules. As the process of shortening and thickening progresses, these become, so to speak, telescoped together, and the end view of each filament exhibits four chromatin masses corresponding to the four rows just described, and which thus appear as tetrads similar to those of Cyclops, although they would appear to have a very different origin. For whereas in the latter case the single units of the tetrad have arisen as the consequence of two longitudinal fissions of the original chromatin granules, in the case of Cyclops the same appearance is apparently produced partly by a longitudinal fission, and partly by a transverse delimiting of the original granules. In the first maturation division of the egg of the same animal, each chromosome is seen to be divided completely, into four segments, 32 THE STRUCTURE OF GEE IEES though it is not certain as to whether the details of their develop- ment are similar to that of the spermatogenetic tetrads as described by Brauer. Meanwhile, other changes have been proceeding, both within and without the nucleus. The nucleolus commonly can be seen to lose substance during the growth of the chromosomes, as is testified by its vacuolated appearance. Often it fragments into smaller particles during the prophases. But it has been definitely ascer- tained, in many cases, that some part of this body is cast out into the cytoplasm where it degenerates, and it is not improbable that this will prove to be of very general occurrence. ‘The possible significance of this event ought not to be overlooked in view of its striking occurrence in the lower forms of life, for it is in them that the clue to the meaning of the complex changes observed in the higher animals and plants must probably be sought. In the cytoplasm, also, remarkable changes connected with the centrosome and spindle mechanism have been proceeding. The latter reaches very different degrees of completeness in different organisms, and, as has been said on a previous page, even in the different cells of the same organism. In the simplest case the two centrosomes, when present, diverge and form a spindle not dis- similar from that already described for somatic nuclear division. In other instances (¢.g. in Salamander) the two centrosomes diverge tangentially to the nucleus, and the spindle is formed between them, and, in the first place, without immediate reference to the nucleus. Later on, however, from the poles of the central spindle thus differentiated not only do the radiating fibrils reach into the protoplasm, and even to the periphery of the cell, but there are also others that extend to the nucleus and become attached to the chromosomes. The latter are thus, as it were, roped up and pulled on to the periphery of the first formed spindle (Fig. 15, D, £). Almost every gradation occurs between the extreme forms here sketched, and the matter seems to be essentially one of more or less complete division of labour between the constituent parts of the achromatic spindle regarded as a whole. In the Salamander, and those other cases in which it appears in a more or less complete form, the central spindle seems to function as a sort of support to keep the two poles apart, and to serve as a sort of railroad along which the daughter chromosomes can be pulled along by the con- tracting peripheral or mantle fibrils that attach them directly to the poles. In the divisions of the higher plants, as has already been ob- served, no centrosomes have been identified with certainty, and the spindle at first starts into existence quite irregularly. It speedily, however, becomes for the most part bipolar, although not unfre- quently isolated fragments of extruded nucleolar substance exert a HEE STRUCTURE (OF CELES 33 very obvious influence on the direction of individual fibrils which may deviate towards and even terminate upon them. When the chromosomes have reached the equator of the spindle they may still preserve the form of rings, tetrads, or more complex shapes, and the method of their fission which leads to the severance of the daughter chromosomes is seldom so clearly longitudinal as is the case in a somatic mitosis. The rings present the greatest difficulty, and there exists a considerable divergence of opinion as to whether their division really corresponds to a transverse or to a longitudinal fission of the whole chromosome. The answer practically turns on the conclusions arrived at as to the path of development followed by the chromosome during the earlier phases ; 7.¢. whether the plane of separation really corresponds to that of the cleavage of the granules, or whether it may not be related to a totally different series of events, and marks the separation of originally bivalent chromosomes as is contended by some observers. In the latter case the complete identity of all the parent somatic chromosomes would be preserved in spite of its apparent loss through the fusion of them in pairs. The separation of the daughter chromosomes would be thus interpreted as not due to the fission of one, but as the segrega- tion of each individual of a pair which had previously become temporarily united. After the separation of the daughter chromosomes and the com- pletion of the anaphase (Fig. 15, G) and telophase, the two nuclei which are thus formed commonly commence immediately to divide once more. Again the reduced number of chromosomes reappears, but the character of the process superficially resembles a somatic mitosis much more closely than the preceding heterotype division, and for this reason the name of homotype was given it by Flemming. In reality, however, there are many other points of difference, besides that of the reduced number of chromosomes. The first, and perhaps the chief, peculiarity lies in the fact that there is good reason for believing that the line of fission of the chromosomes is always predetermined during the prophases of the preceding, the heterotype divisions. This is strikingly seen in the case of Ascaris eggs, in which, during the first maturation division, two of the four rods that together represent one chromosome are distributed to the daughter nucleus as the equivalent of a single daughter chromo- some, whilst at the second mitosis each chromosome emerges as a double (not quadruple) body, and at the metaphase the two con- stituent or collective parts separate from each other as the definitive chromosomes of the next (and final) nuclear generation. Essentially the same course of events obtains in cases of tetrads. When these divide at the equatorial plate the resulting dyads thus formed retreat as the daughter chromosomes, and on the rapidly following homotype division the dyads again reappear in the early 3 34 THE STRUCTURE OF CEEES stages and divide at the equator, each half (monad) forming a daughter chromosome. And an essentially similar condition obtains in at least many other more obscure instances. Thus in Salamander and in Trades- cantia, during the dyaster condition of the anaphase of the hetero- type, each daughter chromosome is seen to be longitudinally divided (Fig. 15, #). This almost certainly is the result of the reopening of a split formed during the prophase of the heterotype, but which has escaped recognition in many cases owing to the great difficulties which these earlier stages present in their investigation. And the fission, thus obvious in the dyaster of the heterotype, provides the daughter chromosomes for the next (homotype) mitosis. It is thus seen that these two mitoses, the heterotype and the homotype, which in animals are the immediate forerunners of the differentiation of the sexual cells, are clearly distinguishable from the preceding ones in several important respects. 1. The appearance of the chromosomes in the reduced number, i.@. they are only half as numerous as in the rest of the nuclei. 2. The long duration of the prophase, and the complex changes and rearrangements, including, probably universally, a preparation for the distribution of daughter chromosomes not only for this but also for the succeeding division. 3. The remarkable forms assumed by the chromosomes upon the spindle. 4. The very general extrusion of nucleolar substance, in rela- tively large quantities, from the nucleus during the prophase. Although probably of importance, it would be as yet premature to speculate on the precise weight to be attached to this phenomenon, but it is suggestive when considered in relation to the course of events described for protozoa. The accompanying figure may serve to render clearer the exact nature of the different views which have been held as to the nature of the processes which are passed through in the reduction divisions. The somatic cell (I. in the figure) is supposed to include two chromosomes, and below are represented, diagrammatically, the various alternative phases gone through during reduction, the corresponding stages being shown in any four figures on the same horizontal line. The series II. and III. represent the events which may be passed through on the assumption of the permanence of the chromosomes, whilst the series IV. and V. correspond to those in which such a permanence is denied. In the former case the two original chromosomes, A, B, remain temporarily united, and their two methods of possible separation are respectively shown. In IV. and V. no continuous identity is claimed for the chromosomes, and the two original ones are replaced by a single new one (C). Considerable difference of opinion exists, then, as to the real THE STRUCTURE OF CELLS 35 nature, as well as the meaning of the events which are thus bound up with the two divisions under consideration. The indications If B IOOGG6{U G With, eX F aeeeieee east News) WMIISS ASS WHI. SSG ZA Fic. 17. Diagrams of Heterotype and Homotype mitoses to illustrate the various possible ways of interpreting the chromosome distribution. afforded by the constancy in number of the chromosomes through the cell-generations of an organism point to a morphological per- manence, and it has been argued that the same chromosomes re- peatedly are re-formed at each mitosis. 36 THE STRUCTORE OF CEEES And since, during the reduction division, there is no evidence of the elimination or degeneration of any chromosomes, it is further urged that each of the apparent units appearing in the prophase of the heterotype division are really bivalent, and represent two chromosomes joined end to end, but otherwise behaving as one. Hacker, who has ably supported this view, believes, in common with many who share it with him, that the tetrad is to be thus explained. The longitudinal fission which divides the bivalent rudiment of the chromosome is succeeded by a more or less trans- verse separation or isolation of the chromatin, which marks the fourfold character of these chromosomes. Consequently each tetrad really represents not one but two chromosomes, and whilst the first (heterotype) division corresponds to the line of longitudinal cleavage, the second (homotype) separates and distributes actual entire chromosomes. Hence it is supposed that a real distribution of entire and diverse chromosomes occurs at the homotype mitosis, whilst the heterotype is essentially similar to a somatic division, and the reduction in number (due to the coherence in pairs) of the chromosomes is only an apparent one. If it could be universally proved to be true, such an explanation would account for many of the remarkable peculiarities which, as has been seen, characterise these divisions, besides affording a very strong support to the theoretical views as to the nature of the mechanism of inheritance advanced by Weismann. But the apparently well-established belief that in other cases the preparation for the two divisions is accom- plished by means of two longitudinal divisions of the chromatic linin militates strongly against conceding the value of a general inter- pretation to the views just sketched in outline. And moreover the facts of amitosis as known to occur in some instances, also, though in a somewhat different way, tell against the permanence of the chromosomes, and consequently against the theories which have been founded on that hypothesis. On the whole, the facts at present before us rather tend to support the view of the brothers Hertwig ; according to them the real significance of the process lies in that sudden quantitative reduction of the chromatin which is a necessary consequence of the rapid succession of the two mitoses in question. It has already been pointed out that the reduction divisions are a common feature to both animals and plants. In the latter, how- ever, there appears to exist a much greater latitude as to the point in the life-history at which they occur. In all the archegoniate series of cryptogams, which includes the mosses, hepaticae, and vascular cryptogams, as well as in all the flowering plants, the re- duction divisions are not immediately connected, as they are in animals, with the formation of the sexual cells, but with the asexual spores from which the generation bearing the sexual organs arises. Thus, after the homotype (or second) division, an indeterminate THE SHROGLURIS (OF CHLES oF number, which may be very considerable, of cell-generations inter- venes between the division in question and the differentiation of the sexual cells. It is true that, as in some of the flowering plants (the embryosac development of the lily, for example), the divisions giving rise to the four spores may be omitted, but the characteristic features of the heterotype and homotype mitosis reappear, although thus postponed, in the first divisions of the nucleus of the embryosac (macrospore). This indeed is a fact of the utmost importance, as emphasising the physiological necessity of the process ; just as it in all probability (from its community to animals and plants) pre- ceded current morphological differentiation, so now if necessary it can override morphological limitations, or at any rate it is not bound up with them. Amongst the lower plants the facts have been tolerably com- pletely elucidated in the case of Fucus, an alga in which asexual spore-formation does not occur. The nuclei of the plant possess the double (somatic) number of chromosomes until the formation of the sexual organs. The oogonium gives rise to oospheres (typically eight, though some may degenerate) by these nuclear divisions. Of these, the first two are respectively heterotype and homotype, and follow on each other with great rapidity, the last mitosis not occurring till after an interval of rest. In some of the desmids, and probably also in Spirogyra, there is evidence to show that the reduction divisions, on the other hand, occur not at the close, but at the beginning of the life-history, with the first segmentation of the fertilised oosphere. But in the majority of these lower organisms information of a precise char- acter is still lacking on the matter. And until our knowledge of the corresponding processes in the lower animals and plants becomes much more complete than it is at present, we can scarcely expect to solve the problem as to the utility or the necessity of the complex events connected with the reduction divisions. Although the higher animals and plants exhibit considerable diversity amongst themselves in the series of changes passed through by the nucleus in division, as well as in the relationships exist- ing between the cytoplasm on the one hand and the cell-wall on the other, they nevertheless agree for the most part in the broader out- lines. The points of similarity are, on the whole, more striking than are the differences, and the latter can often be referred with- out much difficulty to unimportant deviations from a common ground- plan. But although this is the case, the actual meaning of the phenomena, as well as their phylogenetic origin (if there be one), can hardly be grasped or explained by a reference to these forms alone. It is in the study of the lowest forms of life that the key to the solution of cytological problems may be sought for with the greatest hope of success, for amidst the striking diversity exhibited 38 THE STRUCTURE OF CHELS in them an analysis of the processes may render it possible to dis- tinguish between the essential and what is merely accessory, and may indicate the mode and directions in which the structures characterising the higher plants and animals have been elaborated. The essence of the type may perhaps be most clearly gathered from a consideration of the deviations from it. Nevertheless, one is confronted at the outset with difficulties. For although the nuclei of many protozoa are apparently extremely simple, yet in the details of their division they may exhibit considerable complexity, and this not by any means always in the direction followed by the nuclei of higher organisms. And conversely, nuclei are not seldom met with in these low organisms which surpass those of the metazoa and meta- phyta in differentiation, whilst in mitosis they are commonly simpler. The features which seem to be common to all nuclei are—(1) the existence of chromatin in some form or other ; (2) a matrix in which the chromatin is imbedded, but which in the simplest cases may be indistinguishable from the ordinary cytoplasm. Furthermore, the fission of the chromatin is a common, perhaps invariable, antecedent to nuclear division, but it is often difficult to ascertain, and may possibly be really absent in some cases ; for example, in many amitotic divisions. The subsidiary structures, amongst which the centrosome stands pre-eminent, can only be rightly appraised when their origin has been traced in the lowest forms, in which various bodies which appear to possess functions analogous with those credited to centrosomes have often been distinguished. As regards the occurrence of nuclei in the Protozoa and the simplest plants, the investigations of recent years have tended to reduce the number of those from which nuclei were formerly believed to be absent, and at the same time it has become evident that the structure in question may be present in very different degrees of completeness. Thus in Chromatium, and probably in bacteria generally, it is not possible to speak of the existence of a definite nuclear body, but granules which on good grounds have been identified with chromatin are to be distinguished in the protoplasmic framework of the cell. In the cyanophyceae also similar granules are visible, but are restricted to a definite specialised part of the cell-protoplasm, although the latter cannot be spoken of as a nucleus. In many of the forms which possess scattered chromatin granules there is visible in the cell a body which is obviously connected with the mechanism of chromatin distribution, for on cell-division the granules of the latter congre- gate around the central body, which sooner or later divides, each half carrying with it half the chromatin, in the form of attendant satellites, to each daughter cell. In some organisms, e.. Tetramitus, the granules are distributed through the cell- THE STRUCTURE OF CHELS 39 protoplasm in the periods intervening between two fissions, and are only intimately associated with the central body at the time of cell- division ; in others again, ¢.g. Chilomonas, they remain constantly grouped in the vicinity of the body to which reference has just been made. No structure has been certainly made out in it, but it has often been compared to, or identified with, the central body present in many of the more highly differentiated Protozoa, such as Euglena, and it has further been likened to the nucleolus and also to the centrosphere of those cells in which these structures have ch. ch Fic. 18. Tetramitus. A, resting cell. B, early phase of division. c.b, central body; ch, chromatin granules. (After Calkins.) been found to occur. Indeed, it would seem that there is at least some justification for the latter comparison, inasmuch as it appears at least to discharge functions somewhat similar to those performed by the centrosome though in a very rudimentary degree. A distinct advance in differentiation is reached when the chromatic and other constituents of the nuclear apparatus are not only aggregated together, but are also delimited from the rest of the cytoplasm by a wall or membrane. The degree of individuality thus obtained provides a condition favourable to further special- isation, but it seems clear that at any rate the linin framework in which the chromatin is imbedded may be fairly traced back to a 40 THE STRUCTUORE OF CLEES true cytoplasmic origin, however much it may have become modi- fied or altered under more special conditions. The chromatin in these primitive nuclei is often aggregated into clumps as in Actino- sphaerium, Noctiluca, Coccidium, etc., or even concentrated into one mass as in Actinophrysand Spirogyra. These masses have beentermed nucleoli by some writers, but recent investigations tend to show that they really represent composite structures which contain other substances in addition to chromatin. In coccidia, for example, Schaudinn and others have shown that, although the chromosomes are derived from them, there exists over and above the chromatin a considerable mass of substance which is left behind after its exit, and much the same is seen in Amoeba hyalina. Possibly their analogues may be sought in certain of the so-called pseudo-nucleoli of the higher organisms, or in those occurring in the nuclei of Spirogyra, where it has been repeatedly asserted that the chromo- somes originate from the nucleolus. As regards the origin generally of the chromosomes and of the peculiar features exhibited by them during mitosis, there seems but little doubt that they have arisen through stages like those seen in tetramitus and chilomonas, in which the distribution of isolated chromatin granules can be followed. The granules first become aggregated into definite tracts, and these form the primordia of the chromosomes themselves. The actual stages passed through are obscure, and even allied species exhibit considerable differences amongst themselves. Thus in the Amoeba, amitosis seems regularly to occur in Amoeba brevipes and A. polypodia, and also in A. crystalligera ; but in A. hyalina, accord- ing to Dangeard, the chromatin separates from a central body and is differentiated to form chromosomes, whilst the remainder of the body gives rise to a rudi- mentary spindle which is entirely intra-nuclear. In A. binucleata the two nuclei divide simultaneously in a mitotic manner, and the same is true of the colonies of the amoeba-like myxo- mycetes. When a _ plas- modium is about to form spores, it may be found with nuclei showing typical we oe oe karyokinetic figures, all the Amoeba binucleate. A, with resting nuclei. B, nuclei being in the same the two nuclei in the aster stage of mitosis. (After Dangeard.) = ( phase. A remarkable dimor- phism occurs in the nuclei of Paramoeba eilhardi, in which Schaudinn describes one of them as resembling a normal amoeban nucleus, THE STRUCTURE OF CELLS 41 whilst the other is much poorer in chromatin. During mitosis the two act as complements, the latter nucleus furnishing the spindle apparatus, whilst the former supplies the chromatin. Much specu- lation has been built on this case, which is assumed by some writers to indicate that the centrosome or centrosphere is equivalent, phylogenetically, to a nucleus. But it may be open to doubt whether the facts in Paramoeba have really been correctly inter- Ces TS AE ea. Bs fire ras 7 te 5! Oe a oa Nat Ms aes Me Sere, , Kn Fic. 20. Acanthocystis aculeata. A and B, forma- tion of the centrosome from nuclear constituents in swarm-spores. C, resting cell. D, nuclear division preceding fission. (After Schaudinn.) preted, in the sense of regarding the spindle-forming structure as a genuine nucleus. Many cases are known in which bodies which represent centro- somes originate from the nucleus, as appears certainly to be the case in Actinosphaerium, and especially in Acanthocystis, as described by Schaudinn. In the latter animal the ordinary nuclear divisions are associated with the fission of a corpuscle or centrosome occupy- ing a central position in the cell, the nucleus lying close to its side. 42 THE STRUCTURE OF CEEES But in the numerous instances in which amitosis occurs (as in budding), the “centrosome” does not divide, and the nucleus of the freshly-budded cell possesses no such structure. Soon after the complete formation of the bud, however, a dense spot is formed within the nucleus, and is then extruded into the cytoplasm, where it continues to function as a centrosphere. The evidence, as drawn from a study of the lower organisms, seems to point strongly in favour of a nuclear origin for the centrosome apparatus in many cases, although the simplest examples cited on a previous page also indicate that, in others, such a structure might be coeval with, if indeed not actually antecedent to, the primary differentiation of a true nucleus. In a considerable number of cases it seems at least clear that the body or the substance which stimulates and brings about the division of a nucleus is derived from the nucleus itself, even though it may migrate into the cytoplasm, where it may continue to exert, under appropriate conditions, that influence on the nucleus which culminates in division. Thus, in diatoms the centrosphere is found in the cytoplasm, just as it exists in many metazoon cells. The actual location of the centrosome does not necessarily, however, settle the question as to its real origin, and it may indeed assume an intra- or extra-nuclear position in closely allied forms ; as, for example, in the two varieties of Ascaris megalocephala, being situated within the nucleus in the variety univalens, and outside it in the variety bivalens. A remarkable side issue has been introduced into the contro- versy as to the phylogenetic origin of the centrosome by a considera- tion of the peculiar nuclear apparatus which is met with in most ciliata and suctoria. These organisms, with a few possible exceptions, possess a mega- and a micro-nucleus, the former presiding over the somatic life and divisions of the animal, the latter only becoming prominent during the phases of sexuality. Some writers have sought to derive, phylogenetically, the centrosome from the micro- nucleus, whilst they see in the meganucleus the representative of the metazoon nucleus. But quite apart, from the fact that, as Schaudinn pointed out, centrosomes appear in the much simpler heliozoa, the fact that the micronucleus alone divides mitotically, whilst the meganucleus always does so by amitosis, seems a serious difficulty in accepting such an interpretation. Moreover, the macro- nucleus itself springs from the micronucleus after each sexual act, and only persists till the close of the sexual cycle, at which period it totally disintegrates, and thus suffers somatic extinction. It would certainly appear that at any rate it is useless to look to such a source for the origin of the centrosome, which really seems to rest on no better basis than a purely fanciful comparison. A consideration of the maturation processes which obtain in te SERUCTURE (OF CEELS 43 the lower plants and in the protozoa when more fully understood will certainly shed light on the obscure phenomena exhibited by’ the higher forms, and may ultimately give the clue for correctly appreciating the general significance of the processes involved. It has already been remarked that the exact point in the life-history at which these remarkable divisions periodically recur is not identical for all organisms, whilst the universality of the process indicates clearly its great and fundamental importance. It has been urged by some that the chromosomes, which are by those writers postu- lated as permanent structures, become distributed between the daughter cells in such a manner that only half of the original number persist in each sexual cell. And in this way room is made for the new ones imported in either of the two conjugating gametes. Others again, like Oscar Hertwig, regard the quantitative reducing of the chromatin (as opposed to that of the chromosomes) as the fact of prime significance. In many of the lower forms, and notably in the coccidia, the evidence tells strongly for this view ; for in them it is the definite fact that a large part of the mother nucleus is left unused when the gametes or gametocytes are produced, and thus there is a quantitative reduction of a very pronounced character. Again, in the same organisms the multiple division of the nucleus, taken together with the amitotic division of the nucleus of the zygote at segmentation, seem to tell equally against a mechanical necessity for similarity in the chromatic strands. It is not easy to believe in the permanent existence of specific chromosomes under such circumstances ; but, on the other hand, there is no doubt that if the different chromatic granules do really represent slightly different structural characters, a qualitative reduction much in the sense assumed by him may actually take place. For there can scarcely exist any doubt but that, as the result of these pro- cesses, the surviving parts of the mother nucleus do not represent ‘(especially after a multiple division) exact images of the original nucleus from which they have sprung. But it would also seem to be clear that whilst both a quantitative and a qualitative reduction have taken place, these can hardly be regarded as direct means of ensuring that an unvarying proportion of the original chromatin shall be distributed amongst the daughter nuclei. Whether the constant proportions observed in the higher forms is to be explained as the result of a more definite constancy in the chromosomes, together with the continuous existence of these structures in the hypothetically more specialised nuclei of the higher animals and plants, is a matter upon which it is as yet impossible to make any positive statement. It may, however, be confidently asserted, having regard to the extraordinary diversity which prevails in the details of nuclear transformations in these lower forms, many of which will be found described in the present volume, that amongst them, 44 THE STRUCTURE OF CELLS if anywhere, are to be sought the clues to the complex though less variable processes which characterise the higher animals and plants. The phenomena of the sexual union of germ cells, and of their contained nuclei, for the most part are, as yet, hardly susceptible of detailed explanation, but there can be no doubt but that chemiotaxis is the proximate factor chiefly concerned. This is beautifully shown in the case of Halidrys, one of the Fucaceae, in which the large eggs attract numbers of sperms which seek to penetrate the egg. Immediately after an entrance has been effected by one of them, it is seen that the egg changes in important respects. It shrinks, and the supernumerary sperms instantly cease their endeavours to enter its substance. On the contrary, they swim rapidly away, and from the surface of the egg a substance is seen to be excreted which probably exerts the repellent influence in question. Indeed, so strong is its action that those sperms which have not quitted the surface of the eggs are rapidly paralysed and killed. The remarkable paths followed by the male nucleus in the egg has been studied by many observers, and there can be but little doubt that here also a specific attraction of some sort effects the final union. As to the significance of the fusion, the evidence at present available points in no certain direction, nor can any of the hypotheses which have as yet been advanced to explain it be regarded as affording satisfactory solutions of the problem. It has been assumed that by its means a sort of rejuvenescence is effected. But this idea, which is not very clear in itself, fails to take the many subsidiary but still general recurring circumstances into con- sideration. Moreover, it is difficult to see why a similar explana- tion should not also cover those vegetative fusions common in endosperm cells and in certain fungal hyphae, but these have never been regarded as constituting sexual acts. And indeed it would appear that the actual initiation of segment- ation in an egg is not necessarily dependent on the fusion or even the presence of two nuclei. Boveri's observations on the fertilisa- tion of enucleated fragments of eggs with sperms, and still more those of Loeb, who succeeded in causing the eggs of sea-urchins to segment parthenogenetically by treating them with magnesium chloride, indicate that the matter is of far greater complexity than a study of the normal occurrences would indicate. Again, Nathan- sohn caused parthenogenetic development of the oospheres of Marsilea to take place by keeping them at a sufficiently high temperature. This last observation seems to be one of greater importance, for it suggests that a slight modification of the metabolic processes, in this case effected by the abnormal temperature, may suffice to set the machinery of segmentation in motion; that is, the actual i —_ BAE STRUCT ORE OF CELLS 45 mechanism is already present in the egg, and only an appropriate stimulus (not the importation of a missing half of the machinery) is required to set it in motion. Long ago Boveri suggested that the centrosome rather than the nucleus was the important body the introduction of which starts the process of segmentation, and it may well be that his suggestion, in a modified form perhaps, and without postulating an organisation of the specific excitatory substance in so definite a form as a centrosome, may contain a con- siderable element of truth. Amongst the lower organisms, as Klebs and others have shown, the conditions favourable to the formation of sexual cells can be largely referred to nutritive sources, and this is only another way of saying that a definite stimulus—ultimately working on the living substance of the organism itself—is responsible for the sexual reaction. But such a view of the matter leaves untouched the question of the secondary utility of sexuality as a means of ensuring variation. Indeed, this latter is perhaps best kept distinct from the primary causes and conditions which first made sex not only a_ possible but an inevitable incident in the life cycle of the greater part of the higher organisms. Less obscure than its relations to the phenomenon of sex are those which exist between the nucleus and the life of the cell. Gruber, Nussbaum, Verworn, and others have shown that in protoplasm which has been deprived of its nucleus the vital functions speedily become more or less deranged, and finally cease altogether. Enucleated fragments of a cell or organism ~ fail to regenerate lost parts, and the apparent exceptions that have been met with constantly turned out, on further investigation, to be contaminated with nuclear influence. Enucleated fragments of an amoeba are unable to excrete the substance which normally enables these animals to cling to the substratum, and in other organisms, although food may be ingested, the protoplasm seems unable to digest and assimilate it. In plants Gerassimoff has shown that cells containing chlorophyll but destitute of a nucleus are usually unable to form starch, and are incapable of excreting a limiting membrane over their free surfaces. On the other hand, allusion has already been made to the transformations the nucleus may undergo in connection with the secretory activity of glandular and other cells, and in this connec- tion, no less than in that of regeneration, the nucleus may be affirmed to preside over the metabolism of the protoplasm. The peculiar processes which are extruded by the nuclei of the nutritive cells surrounding the ovum (Ophryotrocha), and the curious simulation of the initial phases of mitosis met with in secretory cells, can hardly be dissociated from the special functions discharged by their nuclei. 46 THE STROCTURKE ‘OF-CELES. Again, that frequent migration of the nucleus to the seat of special metabolic activity, so often illustrated in plants, affords additional indication of an exercise of the same influence. The developing of a lateral outgrowth on a plant hair, one-sided thickening of the cell-wall, the softening of the latter previous to its perforation—all these are commonly preceded by the arrival of the nucleus at the part of the cell about to be affected. And the very facts of mitosis itself, with the profound chemical and physical changes which accompany it, suffice of themselves to prove that the nucleus contains substances which are capable of undergoing rapid and striking changes. And indeed it is perhaps not improbable that it is in that very lability characteristic of the constitution of the complex substances which together make up a nucleus that the supreme importance of this body to the cytoplasm, and through the latter to the organism as a whole, is to be attributed. THE PROTOZOA (continued) SECTION I.—THE FORAMINIFERA ! CLASS FORAMINIFERA Order 1. Gromiidea. 2. Astrorhizidea. 3. Lituolidea. 4. Miliolidea. ). Textularidea. » 6. Chilostomellidea. 7. Lagenidea. 8. Globigerinidea. 9. Rotalidea. 0. Nummulitidea. THE Foraminifera received their name before their nature was understood. The early anatomists, guided by the likeness of many of their tests to the nautiloid shells of the Cephalopod Mollusca, assigned them to this group, and many Foraminifera were included by Linnaeus and later writers in the genus Nautilus. D’Orbigny, in 1826, divided the ‘‘ Cephalopoda,” having chambered shells, into Siphoniferes, with a more or less tubular siphon traversing the ‘series of chambers; and Foraminiferes, in which the chambers are in communication by foramina. The simple character of the organ- isms which secreted these shells was first recognised (1835) by Dujardin, who placed them with Amoeba, and allied fresh-water forms in the group Lhizopoda. The limits of the group Foraminifera, as here understood, are identical with those of the Reticularia, as defined by Carpenter in his Introduction to the Study of the Foraminifera (8). It includes those Protozoa the protoplasm of which secretes a test (or shell), and is protruded in fine thread-like pseudopodia, which branch freely and anastomose with one another, and present no obvious differentiation into ectoplasm and endoplasm. The great majority of the members of the group form a well- defined assemblage of organisms, clearly allied to one another, and distinct from any other division of the Protozoa; but we cannot at present draw with any certainty the limits between the simpler forms here included and some other simple members of the Protozoa. 1 By J. J. Lister, M.A., F.R.S., Fellow of St. John’s College, Cambridge. 47 48 THE FORAMINIFERA Each of the characters by which the group is defined loses in distinctness when followed into this borderland region. ‘The shell, which attains great complexity in the higher forms, is membranous in many of the lower, and in Lieberkiihnia, Diplophrys, and Myzxotheca (Fig. 2) can hardly be said to exist. In Hyalopus the pseudopodia, though branching and pointed, do not anastomose with one another (Fig. 15), and in several of the fresh-water Gromiidae few anasto- moses are found. The filiform nature which distinguishes the pseudopodia of the Foraminifera from the blunt-ended pseudopodia of the Lobosa is a better defining character; but in view of the close parallel which, as will be explained below, the life-history of Trichosphaerium, a member of the Lobosa, appears to present with that of many Foraminifera, its importance in a natural classification may be doubted. A few of the simpler forms live in moor pools and other fresh waters, but the great majority are marine. Most of these are littoral in habitat ; many extend their range to the floors of the deep oceans; while a small group, few in the number of species, but very abundant in individuals, lead a pelagic existence. The Protoplasm presents a uniform character without any obvious separation into an outer and inner layer of different refracting power. It is finely granular throughout, and coarse granules are usually present in the protoplasm contained within the test. . The pseudopodia in some of the lower forms are long and root- like, and extend to great distances, giving off branches in their course, and to such forms Dujardin’s name Rhizopoda is especially appro- priate ; but most members of the group are characterised by pseudo- podia of a different nature (Fig. 1). They are for the most part very slender, and spread from the neighbourhood of the aperture or apertures of the shell in fan-like or sheaf-like groups. The outer surface of the shell is invested by a layer of protoplasm, and from this also groups of pseudopodia originate. The pseudopodia frequently branch and anastomose in their course, and between the points of union of the reticulum which they form, they generally run straight, owing in part to the tension which they mutually exert on one another. Some of the radiating strands form broad bands, branching peripherally, but the majority are exceedingly slender, and the ultimate branches are of extreme tenuity. At the points of union broad expansions of protoplasm are often formed. The network of pseudopodia is in part projected free in the water, in part applied to surrounding objects, and serves at once for the prehension of food, as a peripheral sensory apparatus, and as a means of attachment and of locomotion. When a strand of the reticulum is attentively examined, the granules contained in the protoplasm are seen to hurry along its Fia. 1. Gromia oviformis, Duj. 1, contour of protoplasm contained within the test ; 2, protoplasm renee ng the test 3, extended pseudopodia. (From Shipley and MacBride, after Max chultze, 4 50 THE FORAMINIFERA surface, as though borne by a stream, and two streams of granules flowing in opposite directions, centrifugal and centripetal, are to be seen on opposite sides of the same filament. Sometimes a mass of protoplasm forms a swelling on the surface, and is carried along for some distance before it thins out and merges in the substance of the filament. The tip of a pseudopodium may be seen to be alternately ex- tended and retracted according as the centrifugal or centripetal stream gains the ascendency. A mass more solid than the rest of the protoplasm may be seen to be carried to the tip, turn and pass back for some distance with the return current, then to be caught in the centrifugal stream, and again carried to the tip. One is reminded of a cork at the summit of a jet of water, under the contending forces of the upward flow of the jet and of gravity. But the cause of the streaming movement in the pseudopodia of the Foraminifera, like the ultimate cause of movement in all contractile tissues, is still beyond the limits of our knowledge. The minute structure of the protoplasm has been carefully ex- amined by Biitschli, who finds in the coarser pseudopodia, and in the membranous expansions between them, the alveolar structure which.is present in so many protoplasmic structures. In the fine pseudopodia, however, this is absent, and they appear under the highest powers as homogeneous threads of extreme tenuity, with the small granules scattered along their surface. From the peculiar way in which, in the living pseudopodium, the granules course along the threads, sometimes leaving them for a moment to pass apparently through the open water, Biitschli is inclined to the view, suggested by Max Schultze, that an invisible hyaline layer may invest the visible threads of the Foraminifera in the same manner as the more visible hyaline ectosare invests the axial endosare of the pseudopodia of the Heliozoa. In addition to this intrinsic streaming movement there is a movement of the reticulum as a whole. New pseudopodia shoot out from the central mass, others are shortened and retracted, and the whole system is in a condition of tension and constant move- ment, as becomes evident at once when an attempt is made to draw any part by camera lucida. When a strand of the reticulum gives way a momentary collapse of the neighbouring strands is seen, followed by the rapid lengthening of some strands and shortening of others, resulting in renewal of the tension. The food of Foraminifera consists largely of diatoms and algae, either alive or in a state of decay. In some cases, however, it is of animal nature, for Rhumbler finds that the Globigerinae capture and digest the Copepods which abound at the sea surface, and that the pelagic Pulvinulinae contain the skeletons of Radiolaria as well as of diatoms, which have been taken as food (38, pp. 1 and 2). THE FORAMINIFERA 51 Schaudinn also has witnessed the capture and digestion of a Copepod by Myzotheca (41, p. 25), and finds that Patellina and Discorbina feed on Copepod nauplii and Infusoria as well as on Diatoms (45, p. 182). The pseudopodia of Gromia and Poly- stomella have been seen to exercise a paralysing effect on Infusoria which come in contact with them (M. Schultze). The pseudopodia are exceedingly viscid. When an object which serves as food is entangled it becomes surrounded by protoplasm, and if it is large the strands of the reticulum between it and the shell become thicker and more numerous, and the object is drawn inwards. Whether digestion may occur in the extended protoplasm or only after the food enters the shell is uncertain. ee Fic. 2. Myzxotheca arenilega, Schaudinn. N, nucleus; ¢, the gelatinous test with embedded sand grains. (After Schaudinn, 41.) Contractile vacuoles occur in some of the fresh-water forms (Euglypha, Trinema, Cyphoderia, Microgromia, and Platoum), but they have not been seen in any of the marine genera. The granular bodies which are scattered through the protoplasm are of different kinds. Some are coloured, and confer a red, yellow, or brown colour on the protoplasm when seen in bulk. These are allied in composition to the colouring matter of diatoms (diatomin), and are probably derived directly from the food. Some are fatty; others, apparently proteid in nature, are stained by picrocarmine, and are probably formed in the ascending metabolism of the food. In Orbitolites complanata starch grains are abundant, but their formation is probably dependent on the presence of the parasitic algae (zooxanthellae) which abound in the protoplasm in this species. These algae also live in the pelagic Globigerinae. 52 THE FORAMINIFERA The nuclear characters and modes of reproduction of the Foramini- fera are considered below. The Structure of the Test.—The test is found in its most rudiment- ary condition in Myotheca, where it consists of a gelatinous layer, which may form the whole covering or may contain grains of sand. The shape follows the changing contour of the protoplasm, the pseudopodia break through at any point, and no definite and permanent orifice is formed. In Hyalopus (Fig. 15) the test is more resistent, and may have an oval form and a definite orifice ; but here again the shape varies with that of the contained proto- plasm, becoming arborescent when growing among the crowded stems of algae, and the number of orifices may be indefinitely in- creased (Schaudinn, 43). In most of the Gromiidea the test is chitinous and flexible ; but it has a definite shape, and one or two permanent orifices. Another type of test is found in some Gromiidea, and in all the Astrorhizidea and Lituolidea. The tests are here formed of foreign particles, such as fragments of sand, sponge-spicules, the shells of other Foraminifera, etc., fastened together by a cement, which may be firm or flexible, and consist of chitin or calcium carbonate or ferric oxide. In the Astrorhizidae the walls are thick and soft, con- sisting of mud, or of only slightly cemented sand (Fig. 17, a), while in other cases, as in Saccammina (Fig. 17, 6), the particles are united into a rigid structure. In some species of Textularia, Quinqueloculina, and other genera, though the tests are chiefly calcareous, a large proportion of foreign arenaceous material is contained in them. A very remarkable feature of the tests of arenaceous Foramini- fera is the evidence they appear to offer of a selective power exercised by certain species, in collecting materials. In some cases, no doubt, the nature of the test depends on the constituents of the sea bottom in which the animal lives, but in others certain elements alone are selected. Thus in the same dredgings may be found the tests of Pilulina and Saccammina, the former composed of a close felt of siliceous sponge-spicules, laid together to form a wall of uniform thickness ; the latter of coarse grains of sand united by cement (Fig. 17, ) and c). In both the test consists of a single spherical chamber, and the size attained is about the same in both. The cylindrical tests of Dathysiphon filiformis, Sars, are composed of a felt of sponge-spicules, covered externally by a layer of fine sand. The same sample of Pteropod ooze supplies representatives of species of the family Lituolidae, characterised by the coarseness of the sand grains of which the tests are composed ; and of T'rochammi- nina distinguished by fine-grained tests. The type of test found in Huglypha (Fig. 3) and its allies is very exceptional. It is formed of rounded or hexagonal plates, of siliceous THE FORAMINIFERA 53 or, in some cases, chitinous nature, which are secreted in the sub- stance of the protoplasm in the neighbourhood of the nucleus. to the surface, they are there built together into a regular test. exception of some forms of Biloculina, which have been found when living at great depths with a siliceous shell instead of the normal calcareous one, this is almost the only instance of the secretion by the Foraminifera of a siliceous skeleton—a fact which is all the more remarkable in considera- tion of the prevalence of siliceous skeletons among the Radiolaria and He- liozoa. In most of the other orders of Foramini- fera, though a chitinous element is present in the skeleton, it is only the basis in which car- bonate of lime, together Passing With the Fie, 3. Euglypha alveolata. Two stages in the process of division. In one the nucleus is about to divide, and the plates which will form the new shell are seen in the protoplasm; in the other the division is nearly complete. c.v, contractile vacuole. (After Schewiakoff, 48.) with a small proportion of carbonate of magnesium and traces of other salts, are deposited.! 1 The following analyses are given by Brady (8, pp. xvii. and xxi.) of the tests of two species —one porcellanous, belonging to the Miliolidea ; the other perforate (see footnote, p. 54), a member of the Nummulitidea :— Orbitolites complanata, var. laciniata. Silica 0-14 Carbonate of lime 88°74 Carbonate of magnesia 9°55 Alumina with phosphate of lime and magnesia occasional Alumina and ferric oxide traces 98°43 Amphistegina lessonii. Silica : : 0:30 Carbonate of lime with a little organic matter : - 92°85 Carbonate of magnesia 4°90 Alumina with phosphates of lime and magnesia too Ferric oxide trace 100°00 Sollas has examined the specific gravity of the shells of perforate and imperforate species of Foraminifera (66, p. 374), and finds that in perforate forms it varies from 2°626 to 2°674. In examples of Miliola, Peneroplis, and Orbiculina it varies from 2°7 to 54 THE FORAMINIFERA In these the tests are rigid structures, and communicate with the exterior either by one or more large apertures exclusively, as in the majority of the Miliolidea, or by a multitude of small pores in their walls in addition to the large apertures. In some genera of the Miliolidea the shells have a polished white appearance re- sembling porcelain when seen by reflected light, and a yellowish brown horn colour when seen by transmitted light; in the per- forate forms the tests are more transparent, and are in mauy cases as clear as glass. On this account the forms with perforate cal- careous tests are often known as the vitreous or hyaline Foraminifera. In the perforate forms the pores passing out from the chambers have, on the whole, a direction perpendicular to the walls, and transmit pseudopodia. In some cases the pores are large and com- paratively few, in others they are fine and very numerous. The Growth of the Test—In forms such as Myzxotheca and Gromia the growth of the protoplasmic body is accommodated by the simple expansion of the soft and membranous test. Among the arenaceous forms, the feebly cemented, star-shaped tests of Astrorhiza (Fig. 17, a) increase in size in part by the extension of the test along the protoplasmic trunks forming the rays of the star, but in part, doubtless, by the expansion of the central body. In the case of Saccammina (Fig. 17, b), however, and other forms with rigid single- 2°722. The specific gravity of calcite is 2°72, and that of aragonite 2°97. He con- cluded that the calcareous constituent of the perforate tests is calcite, and that of the imperforate tests either aragonite or calcite together with some other and heavier substance. Cornish and Kendall (12) had previously indicated the conclusion, though without positively stating it, that the porcellanous Foraminifera were composed of aragonite—on the grounds of their opacity, and their appearance in or absence from beds coincidently with Lamellibranch and other fossils which are composed of ara- gonite. Chapman (11, p. 39) also has recently stated that the tests of the Wiliolidea are of aragonite, or rather (following Miss Kelly, Mineralogical Mag. vol. xii. (1900), p- 363) conchite. I am inclined to doubt this conclusion. It appears that the presence of the mag- nesium carbonate (specific gravity 3°056), which Brady’s analysis shows is a larger constituent of the imperforate tests than of the perforate, may cause the higher specific gravity found in the former by Sollas. Moreover, Meigen (22) has recently described a chemical colour test by which calcite may be distinguished from ara- gonite, or from the constituent which Miss Kelly has named conchite. Tried by this test, I find that Miliolina and Orbiculina and Orbitolites do not give the colour reaction characteristic of aragonite, but agree with structures composed of calcite. I am indebted to Mr. A. Hutchinson for calling my attention to Meigen’s paper. 1 In the systems of classification prepared by Reuss (1861) and by Carpenter and his colleagues (1862) the Foraminifera were divided into the two groups, the Imperforata and the Perforata. In the latter classification the group Imperfor- ata includes the Gromida, Lituolida, and Miliolida; the Perforata the Lagenida, Globigerinida, and Nummulitida (i.e. families 5-10 of Brady’s classification, which is followed in this article). The progress of deep-sea investigation since this date has revealed the existence of the Astrorhizidea, some of which have perforate and others imperforate tests. Moreover, even in the Miliolidea the first formed chambers may be perforate in the genus Peneroplis (Rhumbler), and in Orbiculina and Orbitolites, as shown in this article. It thus becomes evident that the presence or absence of perforations in the shell, though perfectly characteristic of many of the orders, cannot be taken as the basis for the subdivision of the whole group. THE FORAMINIFERA 55 chambered tests, the way in which the growth of the protoplasm is accommodated is less obvious. It is probable, however, that here also the shell, though at any particular moment rigid, is slowly moulded and expanded under the influence of the protoplasm. The small tests hitherto classified as Psammosphaera fusca, but regarded by Rhumbler (33) as the young of Saccammina sphaerica, with which he finds them to be connected by all intermediate forms, are built up of fragments of sand placed together irregularly, so that the contour of the young test is rough and uneven. In the full-grown Saccammina test the fragments are placed, as in a well-constructed stone wall, to form an even contour. If, as these facts imply, a change in position of the constituents has occurred during growth, there is no difficulty in accepting the conclusion at which Rhumbler arrives, that it has also undergone expansion as a whole. The alternate hypothesis, that in the course of growth to its full size (3-5 mm.) the protoplasm periodically discards its old shell and builds a new one, appears improbable, and the student of the growth of bone will find no a priori difficulty in admitting that a rigid structure may be the seat of profound inter- stitial changes of substance. The growth of the tests of the cylindrical forms of the Astro- rhizidea is effected by extension in a linear direction, fresh arenaceous material being incorporated at their ends. They form simple or branched (many Hyperamminae, Rhizammina), but usually un- segmented tubes. In Hyperammina subnodosa (Fig. 17, d) the tubes are constricted at irregular intervals, and thus present a transitional condition of structure to the definitely segmented, chambered shells of the great majority of Foraminifera. In the latter, while the growth of the protoplasm as the result of the assimilation of food is continuous, the growth of the shell is not continuous but periodic. When a new chamber is to be formed, a mass of protoplasm is protruded from the mouth of the shell, and at the surface of this the new wall is formed, by secretion in the case of calcareous shells, by cementing together of foreign elements in the arenaceous forms. In some genera the secretion of shell substance takes place only on the free surface of the protoplasm, but in others it occurs also where the protoplasm rests against the previously formed test. In such cases the septa dividing the chambers are double, and the new chamber is complete on all sides with the exception of the aperture or apertures left for communication with the exterior, or with its successor, when a new chamber shall be added. In either case the result of this periodic shell formation is the building up of a segmented test, the segments of which, the chambers, are sharply marked off from one another. Max Schultze found that in the formation of a new chamber by Polystomella, the deposit of calcareous salts began before the chamber had 56 THE FORAMINIFERA assumed its final form. The small pocket-like “ retral processes,” which are characteristic of this genus (see Fig. 7), were not formed until some time after a continuous wall had been secreted, so that a partial absorption and re-deposition of the lime salts must here occur (64, p. 30). Structure and Mode of Growth of the Shell Wall.—After their first formation by secretion at the surface of a mass of protoplasm, the walls increase in thickness by the addition of shell material to the outer surface. The anterior wall of each chamber is soon covered by the addition of a new chamber in front of it, and it then forms the whole, or half (as the septa in the species concerned may be single or double), of the septum dividing the chambers from one another. On the septa of the perforate forms the pores may be limited to the peripheral parts or absent altogether. The thickness attained by the septa is not great, but the part of the wall turned to the outer world continues to grow, and may attain considerable thickness. The thickening results from the addition of successive layers of material on the outer surface, and thus a laminated structure is produced ; but though laminated tangentially, the shell is built up, where it is perforated, of hexagonal prisms disposed radially to the surface, and each traversed by a pore transmitting a pseudopodium. It appears that this result may be explained as follows. The shell material is deposited by the protoplasm traversing each of the pores which perforate the wall, on the area about its orifice. It would appear that at the limits between the areas influenced by neighbouring pseudopodia there is some slight difference in the character of the material secreted, and the result is that the deposit is not quite uniform, but marked out into small hexagonal areas, with a pore at the centre of each. If this is the case, the prismatic structure results from the observance of the same limits in successive layers throughout the thickness of the shell. In the more complicated perforate Foraminifera a system of sinuses or canals is present, the main channels of which run in the substance of the shell, and are distinct from the cavities of the chambers, though communicating freely with them by branches. This is known as the canal system. It is, of course, wholly distinct from the radial pores leading direct from the chambers to the exterior. The details of its distribution in Polystomella are given below (cp. Fig. 9). It will be seen that in this form two main “spiral canals” run on either side of the test, parallel with the series of chambers, and give off branches, some of which run in the thickness of the septa between the chambers, while others pass direct to the exterior in the axial regions of the test. About the ultimate branches of the canal system a deposit of shell substance is laid down, which may be called the canalicular skeleton. In the test of Polystomella this skeleton is mainly limited to the axial regions, THE FORAMINIFERA 57 but in other genera it forms extensive deposits in the interior of the septa, and on the surface of the test. It attains a great development in Calcarina.’ Where the canalicular skeleton comes in contact with that of the chamber walls, the two merge insensibly into one another ; the only distinction between them is that one is penetrated by the branches of the canal system, the other by radiating pores leading direct from the chambers of the test. Some confusion has arisen in the use of the terms intermediate or supplemental skeleton, and proper chamber wall. In the Introduction to the Study of the Foraminifera (p. 50) Carpenter says that the “intermediate or supplemental skeleton ” is “‘ formed by secondary or exogenous deposit ” ; and further, that wherever developed to any considerable extent, “it is traversed by the canal system.” The statement that the supplemental skeleton is formed by a secondary or exogenous deposit appears unfortunate, for the walls of the chambers of all the perforate calcareous forms are at first exceedingly thin, and they increase in thickness by the deposition of shell substance on their outer surface, so that the greater part of the shell in all may be said to be secondary and exogenous. The part of the shell first formed is in most, if not all, cases quite indistinguishable from that which is added later. The second character of the supplemental skeleton given by Carpenter, that it is traversed by the canal system, does, however, touch on a real distinction. Biitschli (6, pp. 26-27) calls attention to the fact that a difference between a primary shell layer (Carpenter’s ‘proper wall”) and a secondary mass is often indistinguish- “eeehliilis |) Syn iii; & mS the shell, whether unperforated, perforated by radial pores, or by branches of the canal system. It appears to be more advan- 1 toon Ee tageous to distinguish the skeleton HEA phiiiead IPs developed in relation with the canal BVERNAN system from that of the chamber wall, and as confusion is attached to the name supplemental skeleton, the term canalicular skeleton is used in this article for the former. Repair.—The power of re- able; but he proposes to use the latter term for the outer layer of OS > qr it pairing injuries is very great, Fi. 4. and indeed a fragment may Specimen of Orbitolites tenwissima in which 2 a fragment of a test has given rise to a new in some cases, give rise to a disc. (From Carpenter, 9, Plate I. Fig. 7.) new individual. This is well seen in the specimen of Orbitolites tenuissima shown in Fig. 4, 1 Cp. Carpenter, 8, p. 216. 58 THE FORAMINIFERA in which it is interesting to note that the centre of symmetry of the growth which occurred after the injury is entirely different from the original one. Verworn (67, p. 455) finds that when a specimen of Polystomella crispa (his observations were doubtless made on megalospheric forms, see below) is broken into fragments, several of the larger pieces remain alive and extend pseudopodia, but new shell is secreted over the broken surfaces by only one, and this is found on examination to be the fragment in which the nucleus is contained. The Form of the Test.—The principal forms of test met with among the Foraminifera will be considered later. We shall see that in some genera a particular mode of growth, in relation to some simple symmetrical plan, whether rectilinear, plano-spiral, helicoid, or annular, is observed with perfect regularity, while in others a symmetrical plan is only loosely followed. Many species are adherent to other objects, and in them the chambers may be “heaped” together irregularly, forming what are known as the “acervuline” tests.1 Some of the adherent forms take on an arborescent shape. Multiform Tests—A remarkable phenomenon is presented by many genera, and that is that the plan on which the chambers are arranged in the growth of the test changes in the course of growth. In such tests the chambers which succeed the central one are arranged on a particular plan, whether plano-spiral, helicoid, or some other, and after growth has progressed on this plan for some time a change occurs, and a new plan of growth is with greater or less abruptness adopted (Figs. 24, 39, 40, 44, etc.). In some cases the plan of growth may be changed more than once before the test is completed. Thus two or more types of arrangement of the chambers are, in these genera, presented by the same test at different stages of its growth. The genus Spiroplecta is an example in which the chambers are at first uniserial and arranged in a plano-spiral, and later biserial and in a rectilinear series (Fig. 44, A and B). To tests exhibiting such different modes of growth the terms dimorphic, trimorphic, and polymorphic (according to the number of forms of growth present) were originally applied, and the phenomenon of their occurrence was spoken of as dimorphism, trimorphism, or polymorphism. But it has since been discovered that two kinds of individuals occur in the life-cycle of many For- aminifera, and for this, which is, of course, an entirely distinct phenomenon, the term dimorphism has, in accordance with customary biological usage, been adopted. It has thus become necessary to find other terms to characterise tests displaying two or more modes of growth, and the adjectives biformed and triformed may, as proposed by Rhumbler (36, p. 63), be con- 1 Acervus, a heap. THE FORAMINIFERA 59 veniently used for this purpose. In what follows I have used the term multiform to cover any departure from the uniform condition of growth. It is shown below that while in some genera both forms of individual which are found in one species are alike bi- or tri-formed, Fia. 5. Block of Eocene limestone, showing the two forms of a species of Nummulites, constituting “‘a pair”; the larger named N. biarritzensis, the sinaller N. guettardi, d’Arch. Specimen from Deir en Nakhl, Egypt, in the Brady Collection, Cambridge. in others the phenomenon is exhibited by only one form (the microspheric), or exhibited by it to a greater degree. The Phenomenon of Dimorphism. This phenomenon was first recognised in the fossil nummu- lites which abound in the marine deposits of the Eocene period, and are represented by a single species living at the present day. 60 THE FORAMINIFERA They are often so abundant that, as in the rock of which some of the Egyptian pyramids are built, their coin-like shells, whole or in fragments, constitute the main part of the deposit (Fig. 5). The shells are, in reality, not flat but biconvex discs, and the chambers, arranged in a spiral, are so disposed that the greater part of the cavity of each lies in the median plane, while the shell on either side of this plane is comparatively solid. They thus readily break, as the result of weathering or by artificial means, into plano-convex halves, which display a section of all the chambers from the centre to the periphery on their broken faces. It has long been recognised that while the great majority of the specimens of nummulites occurring in a deposit attain a certain, moderate size, a few are found scattered through it, whose diameter far exceeds that of the others. On examining median sections of the smaller specimens it is usually found that the spiral series of chambers starts from a large and nearly spherical chamber readily visible to the naked eye, and occupying the centre of the shell, while in the larger specimens the spiral series is continued to the centre, where, in carefully prepared sections, it may be seen to take its origin in a spherical chamber of micro- scopic size (Fig. 6). Although the two forms were thus found to be associated in the same beds, and to agree with one another closely except in the size to which they grow and the characters of the central chambers, they were given separate specific names, and attention was called to the puzzling occurrence of these associated pairs of species, a large and a small one, in various deposits. Thus the names Nummulites laevigata, Lam., and N. lamarcki, d’Arch., have been given to two associated forms occurring in beds of the Middle Eocene formation. The former attains a diameter of 20 mm., while the latter does not exceed 3 or 4 mm. Small examples of V. laevigata are not to be distinguished by external characters from the associated form, but on splitting them open, the difference in their central chambers is. at once apparent (Fig. 6). Sixteen pairs of similarly associated “species” belong- ing to the genera Nummulites and Assilina have been enumerated. The possibility that the two associated forms might belong to the same species was, however, entertained by several observers, and the acceptance of this view was accelerated by Munier- Chalmas (26), who (in 1880) definitely formulated the conclusion that the species of nummulites are dimorphic, each appearing under two forms, a large one and a small one. He also expressed the opinion that the phenomenon of dimorphism would be found to be of general occurrence among the Foraminifera. As already stated, the large forms with a small central chamber are much less abundant than the others, and it so happened that THE FORAMINIFERA 61 young individuals of this form did not come under Munier- Chalmas’s observation. He was thus at first inclined to the view that the individuals of the two sets, although in some way dis- tinct in nature, began life under one form, namely, that with a large central chamber. At a certain stage, it was supposed, the growth of one set of individuals was arrested, while in the members of the other set the walls of the large central chambers were absorbed, and growth was continued not only by the addition of chambers at the periphery in continuation of the series of those already formed, but also in a centripetal spiral towards the centre Fig. 6. Nummulites laevigata, Lam. A, Central portion of a section of the megalospheric form (‘(N. lamarcki,” d’A.); B, of the microspheric form. Both x 10. (After de la Harpe, 17.) of the shell, filling the space originally occupied by the large central chamber. This idea of the relationship of the two forms was controverted by de la Harpe, who pointed out, expressing his own views and those of de Hantken, that young examples of the form with a small central chamber are known to occur, and also that differ- ences may be detected not only at the central parts of the shells of the two forms of nummulites, but throughout the series of chambers. Thus it is often found to be the case that in the forms with a large central chamber (A, Fig. 6) the maximum size of the chambers subsequently added is attained early in the series of whorls, while in the others (B) the size of the chambers gradually increases to the last whorl. While the view that one form results from the modification of the other was thus shown to be untenable, it was suggested that 62 THE FORAMINIFERA they might with more reason be regarded as representing the two sexes of a species. The authors did not, however, abandon the old idea of the specific distinctness of the two forms. Investigation of other genera of Foraminifera has shown that the phenomenon of dimorphism is, as Munier-Chalmas expected, widely found among them. The relation between the two forms will be best elucidated by examining the structure and life-history of a single species. For this purpose we will select Polystomella crispa (L.), the life-history of which is most completely known. THE STRUCTURE OF POLYSTOMELLA CRISPA.—This is one of the most abundant of the littoral Foraminifera. It lives in shore pools and down to a depth of 355 fathoms, and ranges from Green- land in the north and Kerguelen Island in the south to the equator. It is very common on our own shores. The test is biconvex and symmetrical about the median plane. The chambers are arranged in a spiral series, and are equitant, 7.¢. they bestride the chambers of the preceding convolution and over- lap them at the sides, each being prolonged in what are known as alar prolongations, which extend towards the spiral axis of the test. Partly as the result of this overlapping, and partly because the axial region is filled in with canalicular shell substance, only the last convolution of chambers is visible externally. On the terminal face of the last chamber, where this face joins the wall of the preceding convolution, a V-shaped line of pores (Fig. 7, a) is visible. These represent the aper- ture of the test, and are the main channels of communication between the terminal chamber and the exterior. At the posterior margin (i.e. the margin remote from the terminal face) of each chamber a number of Fie. 7. pocket-like prominences, The test of Polystomella crispa (L.). x about 40. the retral processes (Figs. a, the line of terminal apertures; 7, retral pro- cesses ; between them are seen the pits by which Tj ana § r) project back- branches of the canalsystem communicate with the ? ? exterior. wards, and are marked by ridges on the external surface. They end blindly, but are separated by pits, at the bottoms of which are the openings of branches of the canal system, which will be described later. The outer surface of THE FORAMINIFERA 63 the shell is dotted over with minute tubercles (not visible in Fig. 7). A keel-like thickening runs round the margin of the test, and in some specimens small spines, like the points of a spur, project from it at the places where the septa join the keel. These are more frequently present in the earlier than in the later convolutions. The pores traversing the walls of the chambers are in this species exceedingly minute. They are hardly visible when the test is seen from without, but they may be detected when a broken piece of the wall is highly magnified and seen by transmitted light. On examining the external characters of the tests of a number of Polystomellas, they are found to form a uniform series, presenting such gradations of size from small to large as may be seen, for example, in a sample of the shells of any Molluse which contains young and old. If, however, a batch of living Polystomellas is killed by some reagent which dissolves the shell but preserves the protoplasm filling its chambers, the protoplasmic casts of the shells no longer form a uniform series but fall into two sets (Fig. 8). In the great majority of them the series of chambers, when traced to the centre of the spiral, is seen to take its origin in a large spherical chamber, having a diameter generally between 60 and 100 ». In the others a small central chamber, with a dia- meter of about 10 p,' occupies the centre of the test, and the suc- ceeding chambers of the series are at first correspondingly small, so that for a given diameter of test these specimens have a greater number of chambers than the others. It is clear that though in Polystomella there is no marked differ- ence in the size attained by the two forms, we have here the same phenomenon of dimorphism which is exhibited in the nummulites. The large central chamber is known as the megalosphere, the small one as the microsphere, and the two sets of individuals of the species are known as the megalospheric and microspheric respectively. The numerical proportion of the two kinds of individuals probably varies with the season, but the megalospheric form is here also always the more abundant. In a large batch of several hundred specimens the megalospheric forms were found to be thirty-four times as numerous as the microspherie. In the protoplasmic casts obtained in this manner the form 1 The diameter of the microsphere varies in the specimens of 7. crispa which I have seen from 6°5 to 13 uw. ‘That of the megalosphere from 165 to 35 u. These dimensions fall, however, a little short of the actual diameters of the chambers, owing to the shrinkage of the protoplasm produced by the reagents. When comparing the size of the microsphere in specimens preserved in this manner and in those examined by sections of the test, it is well to bear this cause of difference in mind. The number given as the diameter of a central chamber, in this article, is to be understood as the mean between the long and short diameters as presented for observation in the specimen. 64 THE FORAMINIFERA and disposition of the chambers are well displayed. In the Polystomella crispa. A, the megalospheric, B, the microspherie forms, decaleified. b, the central chambers of the latter more highly magnified. The canal system is omitted in these figures for the sake of clearness. 7, retral processes ; st, communications between the chambers. megalospheric form the retral processes characteristic of the genus are present in the chamber following the megalosphere (Figs. 8, A, THE FORAMINIFERA 65 and 11,¢c). In the first convolution of chambers the alar prolonga- tions are hardly formed, but as the series is followed on, they project more and more at the sides, overlapping the chambers of the preceding convolutions ; and as they increase the number of apertures between successive chambers, single in the earlier chambers, also increases (Fig. 8, A, Fig. 9, st), so that in the terminal chamber there is, as we have seen, a V- shaped row of pores leading to the exterior. In the microspheric form the arrangement is similar in the later chambers, but in this form the retral: processes are absent from the chambers of the earlier convolutions (Fig. 8, 0). The main trunks of the Canal System lie in the um- bilical region.? They consist of a spiral canal, on either side of the test, running parallel with and just internal to the lateral margins of the cham- . bers—whether these are pro- duced into alar prominences or not (spc, Figs. 9 and 11, c). Opposite the inter- vals between the chambers Fic. 9. meridional canals (m.c) are given off, and run in the thickness of the septa, at some little distance from their outer margins, to meet one another in the median plane. From these numbers of short canals pass outwards, Diagram of a section through a megalospheric example of Polystomella crispa. It is represented as passing through the megalosphere but between the other chambers, in order to show the disposition of the canal system. M, the megalosphere; m.c, meridionial canal; 7.pr, retral processes ; sp.c, spiral canals ; st, protoplasm traversing the apertures be- tween the chambers. The dotted portion indicates the protoplasm filling the chambers, but the canal system is represented as empty. The numerous minute pores leading direct from the chambers to the exterior are omitted, and the shell substance is : left blank. and, in the case of the outer convolution of chambers, open to the exterior, into the pits 1 The canal system is said by Carpenter to be imperfectly developed in Polystomella crispa (8, p. 282), and Mobius (25, p. 103) places P. craticulata, Fichte and Moll, in a new genus Helicoza, on the ground that it possesses a branched canal system, absent in Polystomelia. In decalcified specimens of P. crispa, however, whose protoplasm has been coloured dark by osmic acid, it is easy to convince oneself of the existence of the system, 66 THE FORAMINIFERA between the retral processes, before mentioned as visible on the surface of the test (Fig. 7). The short canals springing from the meridional canals of the inner convolutions open into the chambers of the convolution next external. The canal system is thus in communication with the chambers. In addition to the meridional canals, other branches spring from the spiral canals and pass to the surface in the umbilical region, traversing the thick mass of canalicular skeleton there deposited. The course of the spiral canals is in some cases irregular, and they often break up into a network of sinuses. In the small specimen shown in Fig. 11, ¢, the spiral canal of one side is seen close to its origin, and a meridional canal is shown, between the second and third chambers; but I have been unable to trace the points of origin of the spiral canals. On treating a batch of decalcified specimens with a stain such as picrocarmine, the nuclei appear, and again the two sets of individuals come into marked contrast. The megalospheric form possesses a single large nucleus, while the microspheric form possesses a number of small nuclei, distributed through its chambers (Fig. 8). In Polystomella crispa, then, the megalospheric individuals are numerous, they have a large central chamber and a single large nucleus; while the microspheric individuals are com- paratively scarce, they have a small central chamber, and many nuclei. The existence of the phenomenon of dimorphism being verified, the question arises : How are the two forms related ? For an answer to this question we turn to the life-history, and what is known on this head will now be given. LIFE-HISTORY OF POLYSTOMELLA CRISPA—The Microspheric Form.—The youngest specimens of this form that have been met with already contained many nuclei. Thus in one, described by Schaudinn (44, p. 92), with nine chambers, twenty-eight nuclei were present. The nuclei are at first homogeneous bodies, but as the animals grow nucleoli make their appearances. The nuclei are irregularly scattered through the protoplasm, though they are not found in the terminal chambers. They are often grouped in pairs, and there is good evidence that they multiply by simple division (20, p. 419). The nuclei in the larger chambers are larger than those in the small chambers near the centre, and they may attain a diameter of 40-50 p. In addition to these rounded nuclei, there are generally present in the protoplasm of the microspheric form abundant irregular strands of darkly staining substance, which are apparently given off by the nuclei. In some cases no definite nuclei are visible— THE FORAMINIFERA 67 all the stained substance present being in the form of such irregular strands.. Reproduction of the Microspheric Form.—The first indication of the approach of the reproductive phase as seen in the living animal Fic. 10. a-c, stages in the reproduction of the microspheric form of Polystomella crispa. Drawn from photographs of one specimen attached to the side of a glass vessel. is a great increase in the number of the pseudopodia. They are so abundant that when the specimen is attached to the side of a glass vessel and seen by transmitted light, they form a conspicuous milk-white halo about the brown shell (Fig. 10, a). The halo is at first composed of clear hyaline protoplasm, but in a 68 THE FORAMINIFERA short time the coarse brown granules, hitherto contained within the test, begin to pass out, and ultimately the whole of the proto- plasm, emerging from the test, is massed within the area covered by the halo and lies between the test and the supporting surface (Fig. 10, 6). Here, after involved streaming movements, the proto- plasm gradually and simultaneously separates into spherical masses of uniform size. The centre of each is occupied by a nucleus, with an area of clear protoplasm immediately surrounding it. A close network of delicate pseudopodia surrounds the spheres and forms a communication between them (Fig 10,c). In a short time each © i z i ig 7 x Z c 2) ie eles Ls it) 66 < 26 ras }: ¥ , r & e Ee | hess ® So if © Po 8 } Pk % g gh el cao G@ 6G 2 * Fic. 10 (continued). d, later stage in the reproduction of the specimen of Polystomella crispa represented in Fig. 10, a-c. sphere secretes a calcareous shell, a single small aperture being left by which the pseudopodia pass out. After lying in close contact for some hours, the spheres rapidly and simultaneously draw apart from one another, and within half an hour from the beginning of the movement they are dispersed over a wide area, and each becomes the centre of a system of pseudopodia of its own, though for some time they are not completely isolated (Fig. 10, d). The whole protoplasm of the parent is used up in the formation of the brood of young, the shell being left empty. The process, from the first appearance of the halo to the dispersal of the young, is complete in about twelve hours. THE FORAMINIFERA 69 In a short time the protoplasm which lies outside the aperture of each of the spheres secretes the wall of a second chamber of characteristic form, and the young individual is then clearly recognisable in size and shape as the two-chambered young of the megalospheric form (Fig. 11, 0). The nature of the parent which gives rise to this brood of megalospheric young is determined by decalcifying specimens which are entering on the reproductive phase, before the proto- plasm has left the central chambers. In upwards of fifty cases Fie 11 Young megalospheric individuals of Polystomella crispu. a, a group of six, two days after their formation. Four chambers are formed. 6, a specimen with two chambers, decalcified and stained ; N, the nucleus; , irregular stained mass. c, a specimen with nine chambers, simi- larly treated; N and n as inDb; sp.c, spiral canal of the canal system; in this specimen it becomes irregular near the last-formed chamber 3; 7.c, meridional canal. thus examined the centre was found to be occupied by a micro- sphere. We have then, in this process, a transition from the microspheric to the megalospheric form. Schaudinn (44, p. 94), has found great variation in the size of the megalospheres produced by one parent, namely, from 10 to 120 p. He therefore holds that the two forms of Polystomella, though differing in their nuclear characters, are not always distinguishable by the size of the central chambers. He also finds (p. 93) that in some cases the shell of the young megalospheric form is not secreted until the spherical masses of protoplasm have wandered about for a long time. The details of the reproductive process given above are those which I have invariably observed in specimens which were kept in clean sea-water. 70 THE FORAMINIFERA In my first observations the water circulating in the tanks of the laboratory was used; and though I repeatedly saw the protoplasm emerge from the parent shell and break up into spheres, the development did not pursue a normal course. Again and again the spheres, after remaining separate for some hours, fused with one another, and finally the mass broke up into irregular globular bodies of very unequal size, which remained alive for days, but did not, in most cases, secrete a shell. It was not until fresh sea-water was used in the jars that I had the pleasure of witnessing the normal process of development. In view of this experience, and of the fact that though I have ex- amined some thousands of specimens of Polystomella, I have never met with a megalospheric form with a central chamber less than 34 p in diameter, I am inclined to think that the great irregularities in size in the brood of young, and the small diameter of some of them, are the result of abnormal development. In some genera, however, as stated below, the two forms cannot always be distinguished by the size of the central chamber. Nuclear Changes.—W hen, in the reproduction of the microspheric form, the megalospheres first become isolated, the centre of each is occupied by a sharply-defined, round nucleus, about 7-8 » in diameter, staining uniformly pink in picrocarmine. At this stage there is also diffused in the protoplasm a material taking a stain in this reagent; but in a short time the stained material, pre- viously diffused, becomes aggregated in defined but irregular masses, which gradually draw together, and for a time frequently hide the nucleus from view. When two or three chambers of the young test have been formed the nucleus is again distinctly visible (Fig. 11, b, V), together with one or more irregular masses (7) formed by the closer aggregation of the previously diffused material. In many cases, though not in all, a mass apparently identical with the latter remains visible in or near the central chamber in specimens of the megalospheric form in advanced stages of growth; but it is, I believe, the round nucleus which was seen in the megalosphere at its first formation which becomes the nucleus (‘‘ principal kern”) of the megalospheric form. The relation between this nucleus and the nuclei and irregular strands of the microspheric parent has not been followed.? Growth and Reproduction of the Megalospheric Form.—As the individuals of this form grow, and the number of their chambers is augmented, the nucleus likewise increases in size, and, leaving the megalosphere, it moves on through the chambers, becoming temporarily constricted as it passes through the narrow passages connecting them. In specimens fairly advanced in growth the 1 Thave not met with any evidence in support of Schaudinn’s statement (44, p. 95) that the large nucleus (“principal kern’’) of the megalospheric form results from the massing together of the irregular strands of the microspheric parent, THE FORAMINIFERA ii nucleus is, as Schulze pointed out (64, a), usually found in or near the chamber which is numerically in the middle of the series. In the cells of growing vegetable tissues the nucleus moves towards that part of the cell at which growth is most actively proceeding.! Thus in a growing root-hair the nucleus is found near the tip; in a young stellate hair, it lies at the point of junction of the rays. Its movement towards the regions of activity results, we must suppose, from a certain force impelling it in that direction, and its position when at rest is at the point where the impelling forces, resulting from the activities of the protoplasm in different parts of the cell, are in equilibrium. In the forms of Foraminifera in which the nucleus is single it appears that its position is likewise dependent on the disposition of the protoplasm. Thus in the megalospheric forms of Cycloclypeus and Orbi- tolites complanata the nucleus is found in or near the central chamber, where, owing to the cyclical growth in these genera, the attractions, due to activity at the periphery, are in equilibrium. In the forms with spiral arrangement the nucleus moves on through the series of chambers as growth proceeds. In Polystomella, however, the nucleus of the megalospheric form always lags some distance behind the point at which, judging from the disposition of the bulk of the protoplasm, we should expect the attractive forces to be in equilibrium. At the earliest stage at which it has been recognised the nucleus appears to be a homogeneous body. As it grows, round nucleoli make their appearance, and these are comparatively large in young specimens, and decrease in size while they increase in numbers as growth proceeds. Sections through the nucleus of specimens fairly advanced in growth show a well-defined nuclear wall, a reticulum with finer or coarser meshes occupying the interior, and rounded nucleoli at the nodal points of the reticulum. Minute granules may often be detected in the strands of the reticulum. It appears that throughout the vegetative phase of the megalo- spheric form small fragments are separated off from the nucleus, and they may often be seen as irregular bodies, sometimes con- taining nucleoli, lying in the neighbourhood of the nucleus, or in the chambers through which it has passed in its passage onwards from the megalosphere. Towards the end of the vegetative phase of the life-history of the megalospheric form the nucleus loses its regular outline and its power of receiving stains, and finally disappears. In such specimens it may often be observed that additional passages have been opened up, by the dissolving action of the protoplasm on the shell substance, between adjoining chambers of inner and outer convolutions, so that the inner chambers are placed in 1 Cp. Haberlandt’s researches, quoted by O. Hertwig, Die Celle und die Gewebe i, p. 259. 72 THE FORAMINIFERA more direct communication with the outer, and so with the exterior. Reproductive Phase of the Megalospheric Form.—In specimens in which the large nucleus has disappeared hosts of minute nuclei may be found, scattered uniformly or in groups through the protoplasm. Presumably these are derived from constituents of the principal nucleus, and possibly of the small nuclear fragments as well, but their precise relation to these bodies has not been followed. When the nuclei are evenly distributed the protoplasm breaks up into minute rounded masses, the centre of each being occupied by a nucleus. Division of these nuclei by karyokinesis now occurs, and is simultaneous or nearly so throughout the organism, and this is followed by a second division of the protoplasm to form rounded bodies about 3-4 » in diameter, each containing one of the daughter nuclei (Fig. 12). Ata later period the contents of the shell issue as active zoospores of approximately uniform size, which swim rapidly by means of flagella. It is to be noted that, as in the case of the reproduction of the microspheric form, the whole of the protoplasm of the parent is used up in the production of the zoospores. Though the zoospores have been seen issuing from the shell, their precise characters, when ripe, have not been accur- ately described ; nor have we as yet direct evidence as to their Section through a specimen of the megalo- fate. spheric form of Fagioaie vipa. oemickes “Theat ena eee between the zoospore and the microsphere is suggested by the fact that there is no great differ- ence in size between these structures, the diameter of the former, before their escape from the parent shell, being 3-4 pw, and that of the latter about 10 ».