The Cell in Development and Inheritance

It is a very interesting and important question whether living protoplasm that appears to the eye to be homogeneous does not really possess a structure that is invisible, owing to the extreme tenuity of the fibrillar or alveolar walls (as was long since suggested by Heitz- mann and Biitschli),^ or to uniformity of refractive index in the structural elements. It is highly prc^bable that such is often the case; indeed, Butschli has shown that such *' homogeneous " protoplasm in Protozoa may show a typical alveolar structure after fixation and staining. This explanation will not, however, apply to the young echinoderm eggs (already referred to at p. 28), where the genesis of the alveolar structure may be follow'ed step by step in the li\ing cell. The protoplasm here appears at first almost like glass, showing at most a sparse and fine granulation ; but after fixing and staining it appears as a mass of fine, closely crowded granules. This may indi- cate the existence of an extremely fine alveolar structure in life; but on the whole I believe that these granules are for the most part coagu- lation-products, since they cannot be demonstrated by staining ijitra vitam, and they very closely resemble the coagulation-granules found in structureless proteids like egg-albumin after treatment by the same reagents. In common with many other investigators, therefore, I believe that protoplasm may in fact be homogeneous dowji to the present limits of microscopical vision.

One of the must beautiful forms of cyto-reticulum with which I am acquainted has been described by Bolsius and Graf in the ncnh ridial cells of leeches as shown in Fig. 19 (from a preparation by Dr. Arnold Graf). The meshwork is here of great di.stinctness and regularity, and scattered microsomes are found along its threads. It Fig. 20. — Spinal ganglion-cell of the frog. [Lknhossek.] The nucleus contains a single intensely chromatic nucleolus, and a paler linin-network with rounded chromatin-granules. The cytoplasmic fibrillae are faintly shown passing out into the nerve-process below. (They are figured as far more distinct by Flemming.) Tlie dark cyto- plasmic masses are the deeply staining " chromophiiic granules" (.N'issi) of unknown function. (The centrosome, which lies near the centre of the cell, is shown in Fig. 8, C.) At the left, two connective tissue-cells.

appears with equal clearness, though in a somewhat different form, in many eggs, where the meshes are rounded and often contain food- matters or deutoplasm in the inter-spaces (Figs. 59, 60). In cartilage- cells and connective tissue-cells, where the threads can be plainly seen in life, the network is loose and open, and appears to consist of more or less completely separate threads (F'ig. 9). In the cells of colum- nar epithelium, the threads in the peripheral part of the cell often assume a more or less parallel course, passing outwards from the central region, and giving the outer zone of the cell a striated appear- ance. This is very conspicuously shown in ciliated epithelium, the fibrill:^ corresponding in number with the cilia as if continuous with their bases (Fig. 17).^ In nerve-fibres the threads form closely set parallel fibrillce which may be traced into the body of the nerve-cell; here, according to most authors, they break up into a network in which are suspended numerous deeply staining masses, the "chromo- philic granules" of Nissl (Fig. 20).- In the contractile tissues the threads are in most cases very conspicuous and have a parallel course. This is clearly shown in smooth muscle-fibres and also, as Ballowite has shown, in the tails of spermatozoa. This arrangement is most striking in striped muscle-fibres where the fibrillae are extremely well marked. According to Retzius, Carnoy, Van Gehuchten, and others, the meshes have here a rectangular form, the principal fibrillar having a longitudinal course and being connected at regular intervals by transverse threads ; but the structure of the muscle-fibre is probably far more complicated than this account would lead one to suppo.se, and opinion is still divided as to whether the contractile substance is represented by the reticulum proper or by the ground-substance.

Nowhere, perhaps, is a fibrillar structure shown with such beauty as in dividing cells, where (Figs. 21, 31) the fibrillae group themselves in two radiating systems or asters, which are in some manner the immediate agents of cell-division. Similar radiating systems of fibres occur in amoeboid cells, such as leucocytes (Fig. 49) and pigment- cells (Fig. 50), where they probably form a contractile system by means of which the movements of the cell are performed.

The views of Biitschli and his followers, which have been touched on at p. 25, differ considerably from the foregoing, the fibrillae being regarded as the optical sections of thin plates or lamelte which form the walls of closed chambers filled by a more liquid substance. Butschli, followed by Rcinke, Eismond, Erlanger, and others, inter- prets in the same sense the astral systems of dividing cells which are regarded as a radial configuration of the lamellae about a central point (Fig. 10, B). Strong evidence against this view is, I believe,

1 The structure of the ciliated cell, as described by Engelmann, may be beautifully demon- strated in the funnel-cells of the nephridia and sperm-ducts of the earthworm. 2 The remarkable researches of Apathy ('97) on the nerve-cells of leeches have revealed the existence within the nerve-cell of networks far more complex and definite than was formerly supposed, and showing definite relations to incoming and outgoing fibrillae within the substance of the nerve-fibres.

afforded by the appearance of the spindle and asters in cross-section. In the early stages of the ^g^ of Nereis, for example, the astral rays are coarse anastomosing fibres that stain intensely and are therefore very favourable for observation (Fig. 60). That they are actual fibres is, I think, proved by sagittal sections of the asters in which the rays are cut at various angles. The cut ends of the branching rays appear in the clearest manner, not as plates but as distinct dots, from which in obHque sections the ray may be traced inwards toward the centro- sphere. Druner, too, figures the spindle in cross-section as consisting of rounded dots, like the end of a bundle of wires, thou^-h these are connected by cross-branches (Fig. 28, F). Again, the crossin^r of

Fig. 21. — Diagram of the dividing cell, showing the mitotic figure and its relation to the cyto- plasmic meshwork. the rays proceeding from the asters (Fig. 128), and their beha^-iour in certain phases of cell-division, is difficult to explain under any other than the fibrillar theory. We must admit, however, that the meshwork varies greatly in differ- ent cells and even in different physiological phases of the same cell ; and that it is impossible at present to bring it under any rule of uni- versal application. It is possible, nay probable, that in one and the same cell a portion of the meshwork may form a true alveolar structure such as is described by Biitschli, while other portions may. at the same time, be differentiated into actual fibres. If this be true the fibrillar or alveolar structure is a matter of secondary moment, and the essential features of protoplasmic organization must be sought in a more subtle underlying structure.^

Space would not sufifice for a comparative account of the endless modifications shown by the cytoplasmic substance in different forms of cells. Many of these arise through special differentiations of the active substance, the character of the structure thus being some- times so highly modified, as in the striated muscle-fibre, that it is difficult to trace its exact relation to the more usual forms. More commonly the cytoplasm is modified through the formation of passive or metaplasmic substances which often completely transform the original appearance of the cell. The most frequent of such modifi- cations arise through the deposit of liquid drops and *' granules " (many of the latter, however, being no doubt liquid in life). When the liquid drops arc of watery nature the cavities in which they lie are known as vacuoles^ which are especially characteristic of the pro- toplasm of plant-cells and of Protozoa. These may enlarge or run together to form extensive cavities in the cell, the protoplasm becom- ing reduced to a peripheral layer, or to strands and networks travers- ing the spaces ; while in some forms of unicellular glands the spaces may form branching canals traversing the protoplasm.

The vacuolization or meshlike appearance arising through the formation of larger vacuoles or the deposit of other metaplasmic material is not to be confounded with the primary protoplasmic struc- ture. When, however, smaller vacuoles or metaplasmic granules are evenly distributed through the protoplasm, a " pseudo-alveolar " struc- ture (Reinke) arises that can often hardly be distinguished from the " true " alveolar structure of Blitschli.^ Comparative study shows that all gradations exist between the "false " and the "true " alveolar structures and that no logical ground of distinction between the two exists.^ We thus reach ground for the conclusion that the coarser secondary alveolar or reticular formations are to be regarded as only an exaggeration of the primary structure, and that the alveolar mate- rial of *Biitschli's structure belongs in the same general category with the passive or metaplasmic substance.^

The centrosome^ is usually an extremely minute body, or more commonly a pair of bodies, staining intensely with haematoxylin and ^ In the latter the alveolar spheres are, according to Biitschli, not more than one or two microns in diameter. ^ This has been demonstrated in the cells of plants by Craio ('96), and more recently by the writer ('99), in the case of echinoderm and other eggs. * The centrosome was apparently first seen and described by Flemming in 1875, ^" ^^^ egg of the fresh-water mussel Anodonta, and independently discovered by Van Beneden, in

some other reagents, and surrounded by a cytoplasmic radiating aster or by a rounded mass known as the attr'actio}i-spJicrc (Figs. 8, 49, etc.). As a rule it lies in the cytoplasm, not far from the nucleus, and usually opposite an indentation or bay in the latter ; but in a few cases it lies inside the nucleus (Fig. 148). In epithelia the centro- somes (usually double) lie as a rule near the free end of the cell (Fig. 21)} There is still much confusion regarding the relation of the centro- some to the surrounding structures, and this has involved a corre- sponding ambiguity in the terminology. We w^ill therefore only consider it briefly at this point, deferring a more critical account to Chapter VI. In its simplest form it is a single minute granule, which may, however, become double or triple (leucocytes, connective tissue- cells, some epithelial cells) or even multiple, as in certain giant-cells (Fig. 14, D\ and as also occurs in some forms of cell-division (Fig. 52). In some cases (Figs. 8, C, 120, 148) the ** centrosome " is a larger body containing one or more central granules or '* centrioles " (Boveri); but it is probable that in some of these cases the central granule is itself the true centrosome, and the surrounding body is part of the attraction-sphere. During the formation of the spermatozoon the centrosome undergoes some remarkable morphological changes (p. 171), and is closely involved in the formation of the contractile structures of the tail.

The nature and functions of the centrosome have formed the sub- ject of some of the most persistent and searching investigations of recent cytology. Van Beneden, followed by Boveri and many later workers, regarded the centrosome as a distinct and persistent cell- organ, which Hke the nucleus was handed on by division from one cell-generation to another. Physiologically it was regarded as being the especial organ of cell-division, and in this sense as the *' dy- namic centre" of the cell. In Boveri's beautiful development of this

the following year, in dycyemids. The name is due to Boveri ('88, 2, p. 68). Van Beneden's and Boveri's independent identification of centrosome in Ascaris as a permanent cell-organ ('87) was quicklv supported by numerous observations on other animals and on plants. In rapid succession the centrosome and attraction-sphere were found to be present m pig- ment-cells of fishes (Solger, '89, '90), in the spermatocytes of Amphil)ia (Hermann. Vto), hi the leucocytes, endothelial cells, connective tissue-cells, and lung-epithelium of salamanders (Flemming, '91), in various plant-cells (Guignard, '91), in the one-celled diatoms (Butschli. '91), in the giant-cells and other cells of bone-marrow (Heidenhain, Van Ban.heke, \ an der Stricht, '91), in the flagellate Noctiluca (Ishikawa, '91), in the cells of marine alg:v (Stras- burger, '92), in cartilage-cells (Van der Stricht, '92), in cells of cancerous growths j epitheli- oma, Lustig and Galeotti, '92), in the young germ-cells as already described, in gland-cells (Vom Rath, '95), in nerve-cells (Lenhossek, '95), in smooth muscle-hbres (Lenhossek, 99), and in embryonic cells of manv kinds (Heidenhain, '97)- •'^'any others have conhrmed and extended this list. Guignard's identification of the centrosomes in higher plants i. open to grave doubt (r/ p. 82). Q- P- 57-

view it was regarded further as the especial fertiHzing element in the spermatozoon, which, when introduced into the fgg, endowed the latter with the power of division and development. Van Beneden's and Boveri's hypothesis, highly attractive on account of its simplicity and lucidity, is supported by many facts, and undoubtedly contains an element of truth ; yet recent researches have cast grave doubt upon its generality, and necessitate a suspension of judgment upon the entire matter. Many of the most competent recent workers on the cytologv of higher plants have been unable to find centrosomes, whether in the resting-cells, in the apparatus of cell-division, or dur- ing the process of fertilization, notwithstanding the fact that undoubted centrosomes occur in some of the lower plants. Among zoologists, too, an increasing number of recent investigators, armed with the best technique, have maintained the total disappearance of the cen- trosome at the close of cell-division or during the process of fertili- zation, agreeing that in such cases the centrosome is subsequently formed dc novo. Experimental researches, also, have given strong ground for the conclusion that cells placed under abnormal chemical conditions may form new centrosomes (p. 306). If these strongly supported results be well founded. Van Beneden's hypothesis must be abandoned in favour of the view that the centrosome is but a sub- ordinate part of the general apparatus of mitosis, and one which may be entirelv dispensed with. Thus regarded, the centrosome would lose somewhat of the significance first attributed to it, though still remaining a highly interesting object for further research.^

The cell-substance is often differentiated into other more or less definite structures, sometimes of a transitory character, sometimes showing a constancy and morphological persistency comj^arable with that of the nucleus and centrosome. From a general point of view the most interesting of these are the bodies known as /A? jr//V/jr ox proto- plasts{Y\g. 6), which, like the nucleus and centrosome, are capable of growth and division, and may thus be handed on from cell to cell. The most important of these are the cJirouiatopJiores or cJironioplastids, which are especially characteristic of plants, though they occur in some animals as well. These are definite bodies, varying greatly in form and size, which possess the power of growth and division, and have in some cases been traced back to minute colourless plastids or

^ Cf. pp. Ill, 304. Eisen ('97) asserts that in the blood of a salamander, Bairachoseps, the attraction-sphere (•' archosome ") containing the centrosomes may separate from the remainder of the cell (nucleated red corpuscles) to form an independent form of blood- corpuscle or " plasmocyte," which leads an active life in the blood. leucoplastids in the embryonic cells. By enlargement and differen- tiation these give rise to the starch-builders (amyloplastids), to the chlorophyll-bodies (chloroplastids), and to other ])igment-bodies (chromoplastids), all of which may retain the power of division. The embryonic leucoplastids are also believed to multiply by division and to arise by the division of plastids in the parental organism ; but it remains an open question whether this is their only mode of origin, and the same is true of the more highly differentiated forms of plas- tids to which they may give rise.

The contractile or pulsating vacuoles that occur in most Protozoa and in the swarm-spores of many Algae are also known in some cases to multiply by division ; and the same is true, according to the researches of De Vries,. Went, and others, of the non-pulsating vacu- oles of plant-cells. These vacuoles have been shown to have, in many cases, distinct walls, and they are regarded by De Vries as a special form of plastid ("tonoplasts ") analogous to the chromatophores and other plastids. It is, however, probable that this view is only appli- cable to certain forms of vacuoles.

The plastids possess in some cases a high degree of morphological independence, and may even live for a time after removal from the remaining cell-substance, as in the case of the "yellow cells" of Radiolaria. This has led to the view, advocated by Brandt and others, that the chlorophyll-bodies found in the cells of many Protozoa and a few Metazoa {Hydra, Spongilla, some planarians) are in reality dis- tinct Alg^ living symbiotically in the cell. This view is probably correct in some cases, e.g. in the Radiolaria ; but it may be doubted whether it is of general appUcation. In the plants the plastids are. almost certainly to be regarded as differentiations of the protoplasmic

The existence of cell-organs which have the power of independent assimilation, growth, and division is a fact of great theoretical interest in its bearing on the general problem of cell-organization ; for it is one of the main reasons that have led De Vries, Wiesner, and many others to regard the entire cell as made up of elementary self-propa- gating units. The structure and origin of the cell-wall or membrane form a subject somewhat apart from our general purpose, since the wall belongs to the passive or metaplasmic products of protoplasm rather than to the living cell itself. We shall therefore treat it very briefly. Broadly speaking, animal cells are in general characterized by the slight development and relative unimportance of the cell-walls, while

the reverse is the case in plants, where the cell-walls play a very important role. In the latter the wall sometimes attains a great thickness, usually displays a distinct stratification, and often has a complex sculj)ture. Such massive walls very rarely occur in the case of animal tissues, though the intercellular matrix of cartilage and bone is to a certain extent analogous to them, and the thick and often highly sculptured envelopes of some kinds of eggs and of various Protozoa may be placed in the same category.

It is open to question whether any cells are entirely devoid of an enclosing envelope; for even in such ** naked" cells as leucocytes, rhizopods, or membraneless eggs, the boundary of the cell is usually formed by a more resistant layer of protoplasm or " pellicle "(Biitschli) which may be so marked as to simulate a true membrane, as is the case, for example, in the red blood-corpuscles (Ranvier, Waldeyer, etc.). Such pellicles probably differ from true membranes only in degree ; but it is still an open question both in animals and in plants, how far true membranes arise by direct transformation of the periph- eral protoplasmic layer (the " Hautschicht " of botanists), and how far as a secretion-product of the protoplasm. In the case of animal cells, Leydig long since proposed ^ to distinguish between " cuticular " membranes, formed as secretions and usually occurring only on the free surfaces (as in epithelia), from *' true membranes " arising by direct transformation of the peripheral protoplasm. Later researches, including those of Leydig himself, have thrown so much doubt on this distinction that most later writers have used the term cuticular in a purely topographical sense to denote membranes formed only on one (the free) side of the ccll,^ leaving open the question of origin. The formation and growth of the cell-wall have been far more thor- oughly studied in plants than in animals, yet even here opinion is still divided. Most recent researches tend to sustain the early view of Nageli that the cell-wall is in general a secretion-product, though there are some cases in which a direct transformation of protoplasm into membrane-stuff seems to occur.

^ In the division of plant-cells the daughter-cells are in almost all cases cut apart by a cell-plate which arises in the protoplasm of the mother-cell as a transverse series of thickenings of the spindle-fibres in the equatorial region (Fig. 34). This fact, long" regarded by Strasburger and others as a proof of the direct origin of the membrane from the protoplasmic substance, is shown by Strasburger's latest work ('98) to be open to a quite different interpretation, the actual wall being formed by a splitting of the cell-plate into two layers between which the wall appears as a secretion-product. Almost all observers further are ao:reed that the formation of new membranes on naked masses of

protoplasm produced by plasmolysis are likewise secretion-products, and that the secondary thickening of plant-membranes is produced in the same way. These facts, together with the scanty available zoological data, indicate that the formation of membranes by secre- tion is the more usual and typical process. ^ The chemical composition of the membrane or intercellular sub- stance varies extremely. In plants the membrane consists of a basis of cellidose, a carbohydrate having the formula CgHjoOg ; but this sub- stance is very frequently impregnated with other substances, such as silica, Hgnin, and a great variety of others. In animals the inter- cellular substances show a still greater diversity. Many of them are nitrogenous bodies, such as keratin, chitin, elastin, gelatin, and the like ; but inorganic deposits, such as silica and carbonate of lime, are common.

In a large number of cases the cell exhibits a definite polarity, its parts being symmetrically grouped with reference to an ideal organic axis passing from pole to pole. No definite criterion for the identi- fication of the cell-axis has, however, yet been determined ; for the general conception of cell-polarity has been developed in two differ- ent directions, one of which starts from purely morphological con- siderations, the other from physiological, and a parallelism between them has not thus far been fully made out.

On the one hand. Van Beneden ('83) conceived cell-polarity as a primary morphological attribute of the cell, the organic axis being identified as a line drawn through the centre of the nucleus and the centrosome (Fig. 22, A). With this view Rabl's theory ('85) of nuclear polarity harmonizes, for the chromosome-loops converge toward the centrosome, and the nuclear axis coincides with the cell- axis. Moreover, it identifies the polarity of the Qg^. which is so important a factor in development, with that of the tissue-cells; tor the egg-centrosome almost invariably appears at or near one pole of

Heidenhain ('94, '95) has recently developed this conception of polarity in a very elaborate manner, maintaining that all the struc- tures of the cell have a definite relation to the primary axis, and that this relation is determined by conditions of tension in the astral rays 1 Strasburger ('97, 3, '98) believes membrane-formation in general to be especially con- nected with the activity of the "kinoplasm," or tilar plasm of which he considers the " Haut- schicht," as well as the spindle-fibres, to be largely composed. In support ol this may be mentioned, besides the mode of formation of the partition-walls in the division of plant- cells, Harper's ('97) very interesting observations on the formation of the ascospores m Erysiphe (Fig. IZ), where the spore-membrane appears to arise directly from the astral rays.

focussed at the centrosome. On this basis he endeavours to explain the position and movements of the nucleus, the succession of division- planes, and many related phenomena.^ Hatschek {^^^) and Rabl ('89, 92), on the other hand, have ad- vanced a quite different hypothesis based on physiological considera- tions. By "cell-polarity" these authors mean, not a predetermined morphological arrangement of parts in the cell, but a polar differen- tiation of the cell-substance arising secondarily through adaptation of the cell to its environment in the tissues, and having no necessary relation to the polarity of Van Beneden (Fig. 22, B, C). This is

A. Morphological polarity of Van Beneden. Axis passing through nucleus and centrosome. Chromatin-threads converging toward the centrosome. B.C. Physiological polarity of Rabl and Hatschek, Zj' in a gland-cell, C'in a ciliated cell. typically shown in epithelium, which, as Kolliker and Haeckel long since pointed out, is to be regarded, both ontogenetically and phy- logenetically, as the most primitive form of tissue. The free and basal ends of the cells here differ widely in relation to the food- supply, and show a corresponding structural differentiation. In such cells the nucleus usually lies nearer the basal end, toward the source of food, while the differentiated products of cell-activity are formed either at the free end (cuticular structures, cilia, pigment, zymogen- granules), or at the basal end (muscle-fibres, nerve-fibres). In the non-epithelial tissues the polarity may be lost, though traces of it are often shown as a survival of the epithelial arrangement of the embryonic stages.

But, although this conception of polarity has an entirely different point of departure from Van Beneden's, it leads, in some cases at least, to the same result ; for the cell-axis, as thus determined, may coincide with the morphological axis as determined by the position of the centrosome. This is the case, for example, with both the spermatozoon and the ovum ; for the morphological axis in both is also the physiological axis about which the cytoplasmic differentia- tions are grouped. Recent researches have further shown that the same is the case in many forms of epithelia, where the centrosomes lie in the outer end of the cell, often very near the surface.^ (Fig- '2-1)