The Cell in Development and Inheritance

The same general conception of the aster is adopted by most of those who accept the fibrillar or reticular theory of protoplasm, the astral rays and spindle-fibres being regarded as actual fibres forming part of the general network. One of the first to frame such a con- ception was Klein i^']'^\ who regarded the aster as due to "a radial arrangement of what corresponds to the cell-substance," the latter being described as having a fibrillar character.^ The same view is advocated by Van Beneden in 1883. With Klein, Heitznian, and Frommann he accepted the view that the intra-nuclear and extra- nuclear networks were organically connected, and maintained that the spindle-fibres arose from both.^ *'The star-like rays of the asters are nothing but local differentiations of the protoplasmic network.-^ . •• . In my opinion the appearance of the attraction-spheres, the polar corpuscle (centrosome), and the rays extending from it, includ- ing the achromatic fibrils of the spindle, are the result of the appear- ance in the egg-protoplasm of two centres of attraction comparable to two magnetic poles. This appearance leads to a regular arrange- ment of the reticulated protoplasmic fibrils and of the achromatic nuclear substance with relation to the centres, in the same way that a magnet produces the stellate arrangement of iron filings."*

This view is further developed in Van Beneden's second paper, pubUshed jointly with Neyt {'^7). " The spindle is nothing but a differentiated portion of the asters." ^ The aster is a ** radial structure of the cell-protoplasm, whence results the image designated by the name of aster." '^ The operations of cell-division are carried out through the " contractility of the fibrillae of the cell-protoplasm and their arrangement in a kind of radial muscular system composed of antagonizing groups." *"

An essentially similar view of the achromatic figure has been advocated by many later workers. Numerous observers, such as Rabl, Flemming, Carnoy, Watase, Wilson, Reinke, etc., have ob- served that the astral fibres branch out peripherally into the general meshwork and become perfectly continuous with its meshes, and tracing the development of the aster, step by step, have concluded that the rays arise by a direct progressive modification of the pre- existing structure. The most extreme development of this view is contained in the works of Heidenhain ('93, '94), Buhler (95), Kosta- necki and Siedlecki ('97), which are, however, only a development of the ideas suggested by Rabl in a brief paper published several years before. Rabl ('89, 2) suggested that neither spindle-fibres nor astral rays really lose their identity in the resting cell, being only modified in form to constitute the mitome or filar substance (meshwork), but still beins: centred in the centrosome. Fission of the centrosome is followed by that of the latent spindle-fibres (forming the linin- network); hence each chromosome is connected by pairs of daughter-

1 It is interesting to note that in the same place Klein anticipated the theory of fibrillar contractility, both the nuclear and the cytoplasmic reticulmii being regarded as contractile (/.r., p. 417). fibres with the respective centrosomes. Heidenhain, adopting the first of these assumptions, builds upon it an elaborate theory of cell- polarity and cell-division already considered in part at pages 103-105. Sometimes the astral rays ('* organic radii ") retain their radial arrange- ment throughout the life of the cell (leucocytes, Fig. 49) ; more com- monly they are disguised and lost to view in the cytoplasmic meshwork. All, however, are equal in length and in tension — assumptions based on the one hand on the occurrence of concentric circles of microsomes in the aster, on the other hand on the analogy of the artificial model described at page 104. Biihler ('95) and Kostanecki and Siedlecki ('97) likewise unreservedly accept the view that besides the centro- some the entire system of '' organic radii," including astral rays, mantle-fibres, and central spindle-fibres, persists in the resting cell in modified form, and is centred in the centrosome. Kostanecki finally ('97) takes the last step, logically necessitated by the foregoing con- clusion, and apparently supported also by the crossing of the astral rays opposite the equator of the spindle and the relations of their peripheral ends, concluding that the monocentric astral system is con- verted into the dicentric system (amphiaster) by longitudinal fiSsion of the rays} Thus the entire mitome of the mother-cell divides into equal halves for daughter-cells ; and since the radii consist of micro- somes, each of these must likewise divide into two.^

Could this tempting hypothesis be established, Roux's interpretation of nuclear division (p. 224) could be extended also to the cytoplasm ; and the aster- and amphiaster-formation, with the spireme-forma- tion, might be conceived as a device for the meristic division of the entire cell-substance — a result which would place upon a substantial basis the general corpuscular theory of protoplasm. Unfortunately, however, the hypothesis rests upon a very insecure foundation : first, because it is based solely upon the fibrillar theory of protoplasm ; second, because of the very incomplete direct evidence of such a splitting of the rays ; third, because there is very strong evidence that in many cases the old astral rays wholly disappear, to be replaced by new ones.^ We may best consider this adverse evidence in connec- tion with a general account of the opposing archoplasm-hypothesis.

Entirely opposed to the foregoing conception are the views of Boveri and his followers, the starting point of which is given by 2 This view had been definitely stated also by O. Schultze in 1890. ^ There is, however, no doubt that the aster as a whole does, in some cases, divide into two — for instance, in the echinoderm-egg. Fig. 95. Boveri's celebrated archoplasm-hypothesis. Boveri has from the first maintained that the amphiastral fibres are quite distinct from the Gen- eral cell-meshwork. In his earlier papers he maintained ('88, 2) that the attraction-sphere of the resting cell is composed of a distinct sub- stance, '' arcJioplasm,'' consisting of granules or microsomes aggre- gated about the centrosome as the result of an attractive force exerted by the latter. From the material of the attraction-sphere arises the entire achromatic figure, including both the spindle-fibres and the astral rays, and these have nothing to do with the general reticulum of the cell. They grow out from the attraction-sphere into the reticu- lum as the roots of a plant grow into the soil, and at the close of mitosis are again withdrawn into the central mass, breaking up into granules meanwhile, so that each daughter-cell receives one-half of the entire archoplasmic material of the parent-cell.

Boveri was further incUned to believe that the individual granules or archoplas- mic microsomes were " independent structures, not the nodal points of a general network," and that the archoplasmic rays arose by the arrange- ment of these granules in rows without loss of their identity.^ In a later paper on the sea-urchin this view underwent a considerable modification through the admission that the archoplasm may not ])re- exist as formed material, but that the rays and fibres may be a new formation, crystallizing, as it were, out of the protoplasm about the centrosome as a centre, but having no organic relation with the gen- eral reticulum ; though Boveri still held open the possibilitv that the archoplasm might preexist in the form of a specific homogeneous sub- stance distributed through the cell, though not ordinarily demonstra- ble by reagents.^ In this form the archoplasm-theory approaches very nearly that of Strasburger, described below.

There are three orders of facts that tell in favour of Boveri's modi- fied theory : first, the existence of persistent archoplasm-masses or attraction-spheres from which the amphiasters arise ; second, the origin of amphiasters in alveolar protoplasm ; and, third, the increas- ing number of accounts asserting the replacement of the old asters by others of quite new formation. In at least one case, namely, that of Noctiluca, the entire achromatic figure is formed from a permanent attraction-sphere lying outside the nucleus and perfectly distinct fn^n the general cell-meshwork.^ Other cases of this kind are very rare, and in most cases the attraction-sphere sooner or later disintegrates,^ but in the formation of the spermatozoa we have many examples of archoplasmic masses (Nebenkern, attraction-sphere, idiozome), which apparently consist of a specific substance having a special relation to the achromatic figure.

The aniphiastral formation in alveolar protoplasm skives very clear evidence aj^ainst the theory of fibrillar persistence. Here the fibrillar rays can be seen growing out lhroii«;h the walls of the alveoli ^ cpiite distinct from, thou«;h embeddeil in, them. At the close of mitosis every trace of the fibrillar formation may disappear, r.i^. in echino- derm-ej;i;s atter formation of the polar bodies, the protoplasm retain- ing only a typical alveolar structure.

A. First appearance of the cleavage-centrosome at the poles of the fused germ-nuclei ; cleavage- astf-rs forming within the degenerating sperm-asters. B. Final anajihase of first cleavage, showing persistent centrosomes and new asters forming. C. Immediately after division. D-F. Three stages of the late anapluise in Thaliisseina, showing formation of new asters within the old. ( Cf. Fig. 99.) The stronr^cst evidence a^^ainst fibrillar persistence is, however, given by recent studies on mitosis, showing on the one hand that the new astral centres do hot coincide with the old ones, on the other that the old ravs degenerate /;/ situ, to be replaced by new ones. Aside from many earlier observers, who believed the entire aster to disappear at the close of mitosis, the first to assert the wholly new

formation of the rays was Driiner, who maintained in the case of the mitosis of salamander testis-cells, that "not a sin^de fibre of the astral system of the mother-cell is carried over unchan<;ed into the orj;anism of the daughter-cell" ('95, p. 309)- The same conclusion was soon afterward supported by Braus ('95) in the case of the cleava<;e- mitoses of Triton. The most convincing evidence of this fact has been given by studies on the maturation and fertihzation of the egg by Grifhn ('96, '99), MacFarland ('97), Lillie ('99), and Coe ('99), all of whom find that the new astral centres, arising by division of the cen- trosome, move away from the old position, to lu/iic/i, hoivcvcr, the old rays still converge ivhile the new asters are independently forming (Fig. 155). This is shown with especial clearness in the i^g'-^ of Cere- brattilus (Coe), where the peripheral portions of the old asters persist until the new amphiaster is completely formed. This observation seems conclusively to overturn Kostanecki's hypothesis of the persist- ence and division of the rays, and together with the work of MacFar- land gives a very strong support to Boveri's later view.

It still remains an open question whether the rays actuallv arise from the substance of the centrosome, from a specific surrounding archoplasm, or by differentiation out of the general substance of the meshwork. The first of these possibilities has been urged in a very interesting way by Watase ('94), who believes that the centrosome "spins out the cytoplasmic filaments"^ of the spindle and aster, and that ordinary microsomes may in Hke manner spin out the fibril la- of ordinary cytoplasmic networks.^ This view is sustained by the mode of origin of the axial filament in the spermatozoa and that of the cilia in plant spermatozoids. It is, on the other hand, opposed by the almost infinitesimal bulk of the centrosome as compared with that of the aster that may form about it, and by the formation of the spindles in higher plants in the apparent absence of centrosomes. On the whole, the facts do not seem at present to warrant the acceptance of Watase's ingenious hypothesis, and the most probable view is that of Driiner and Boveri, that the rays are differentiated out of the walls of the meshwork. In cases where the protoplasm is reticular or fibrillar the differentiation of the ravs mav be indistinguishable from a mere rearrangement of the thread-work ; in alveolar protoplasm they may be seen as new formations, while in either ca.se the material of the old aster may be more or less directly utilized in the building of the new. The feature common to all is the periodic activity either of the centre itself or of the surrounding protoplasm, and the coincidence or non-coincidence of the new aster with the old is apparently a secondary matter.

2 See the same paper for a suggestive comparison of the astral librilLv to muscle-fihrcs In its original form the archoplasm hypothesis, as stated by Boveri, was developed with reference only to the material of the spindle- fibres and astral rays. Later writers have greatly extended the con- ception on the basis of Boveri's earlier view that archoplasm is a specific form of protoplasm, possessing specially active properties. Strasburger ('92-98), whose views have already been considered in part, believes the protoplasm to consist of, or to show a tendency to differentiate itself into, two distinct substances, namely, a specially active fibrillar kijioplasin and a less active alveolar tropJioplasin. The former gives rise to the mitotic fibrillae, constitutes the periph- eral cell layer, or HautschicJit, from which the membrane arises, forms the substance of the centrosomes, and gives origin to the con- tractile substance of cilia and flagella. The kinoplasm is thus mainly concerned with the motor phenomena of the cell, the trophoplasm with those of nutrition ; and this physiological difference is morpho- logically expressed in the fact that the former has in general a fibrillar structure, the latter an alveolar. Beyond this the two forms of protoplasm show a difference of staining-reaction, the kinoplasmic fibrillae staining deeply with gentian-violet and iron-haematoxylin, while the trophoplasm is but slightly stained.

Prenant ('98, '99) still further extends the hypothesis, adopting the view that the " ergastoplasmic " (Garnier) fibrillae of gland-cells ^ are equivalent to the kinoplasmic or archoplasmic fibrillae of the mitotic figure, and to the fibrillae of nerve- and muscle-fibres as well. He is thus led to the conception of a dominating or " superior " cytoplasm (including "archoplasm," "kinoplasm," "ergastoplasm"), which arises by differentiation out of the general cytoplasm, plays the leading role in the elaboration of active cell-elements (" cytosomes "), such as mitotic, neural, and glandular fibrillae, and finally, its role accom- plished, may disappear. Under the same category with the foregoing structures are placed the centrosome, attraction-sphere, mid-body, idiozome, Nebenkern, and yolk-nucleus.

Such a generous expansion of the archoplasm-hypothesis brings it perilously near to a rediictio ad absitrdmn ; for the step is not a great one to the identification of the " superior protoplasm " with the active cell-substance in general, w.hich would render the whole hypothesis superfluous. Physiologically, we can drav/ no definite line of demar- cation between the more and the less active protoplasmic elements, and it may further be doubted whether such a boundary exists even between the latter and the metaplasmic substances.^ It is further quite unjustifiable to infer physiological likeness from similarity in staining-reaction^ or in fibrillar structure. For these reasons the hypothesis of " superior protoplasm " seems one of doubtful utility.

In its more restricted form, however, the urchoplasm or kinopiasm hypothesis is of high interest as indicating a common element in the origin and function of the mitotic fibrilkie, the centrosome and mid- body, and the contractile substances of cilia, flagella, and muscle- fibres. The main interest of the hypothesis seems to me to lie in the definite genetic relations that have been traced between the archo- plasmic structures of successive cell-generations (as is most clearly shown in the phenomena of maturation and fertilization). It has been pointed out at various places in the preceding chapters ^ how many apparently contradictory phenomena in cell-division, fertiliza- tion, and related processes can be brought into relation with one another under the assumption of a specific substance, carried by the centrosome or less definitely localized, which gives the stimulus to division, which is concerned in the formation of the mitotic figure and of contractile elements, and which may be transmitted from cell to cell without loss of its specific character. There seems, however, to be clear evidence that such substance (or substances), if it exists, is not to be regarded as being necessarily a permanent constituent of the cell, but only as a phase, more or less persistent, in the general metabolic transformation of the cell-substance.^

As originally used by Van Beneden ^ the term attraction-spJicrc was applied (in Ascains^ to the central mass of the aster surrounding the " corpuscule central " and consisting of medullary and cortical zones, as already described (p. 310). The cortical zone is bounded by a di.s- tinct circle of microsomes from which the astral rays proceed ; and at the close of cell-division the rays were stated to fade away, leaving only the attraction-sphere, which, like the centrosome, was regarded as a permanent cell-organ. Later researches have conclusively shown that the attraction-sphere cannot be regarded as a permanent organ, since in many cases it disintegrates and disappears. This occurs, for example, in the early prophases of mitosis in the testis-cells of the sala- mander,^ where the sphere breaks up and scatters through the cell as the new amphiaster forms (Fig. 27). A very interesting ca.se of this kind occurs in the cleavage of the ovum in Crcpidiila, as described by Conklin ('99). The spheres here persist for a considerable period after division (Fig. 192), but have no direct relation to those of the ensuing division, finally disappearing /// situ. The new spheres are formed about the centrosomes, which Conklin believes to migrate out of the old spheres (somewhat as occurs in the spermatid, p. iC7)to their new position. The interesting point here is that the old sphere

takes up such a position as to pass entirely into one of the grand- daughter-cells, while the new sphere-substance is equally distributed between them and in its turn passes into one of the cells of the en- suing division.^ In Crepidula, as in Ascaris, the attraction-sphere represents only the central part (centrosphere) of the aster. In some cases, however, e.o. in leucocytes, the entire aster may persist, and the term attrac- tion-spJiere has by some authors been applied to the whole structure. Later workers have proposed different terminologies, which are at present in a state of complete confusion. Fol ('91) proposed to call the centrosome the astrocentre, and the spherical mass surrounding it (attraction-sphere of Van Beneden) the astrospJiere. Strasburger accepted the latter term but proposed the new word centrospJiere for the astrosphere and the centrosome taken together.^ A new complication was introduced by Boveri ('95), who applied the word *' astrosphere " to the entire ^^.y/^-r exclusive of the centrosome, in which sense the phrase " astral sphere " had been employed by Mark in 1881. The word "astrosphere " has therefore a double meaning and would better be abandoned in favour of Strasburger's convenient term centro- spJiere, which may be understood as equivalent to the '' astrosphere " of Fol.

Besides these terms we have Heidenhain's microcentruvi (p. 311), equivalent to the centrosome or group of centrosomes at the centre of the aster, with its surrounding sphere ; ^ Kostajiecki's and Siedlecki's microspJiere, applied to the central region of the aster surrounding the centrosome whether bounded by a distinct microsome-circle or not ; ^ Erlanger's centroplasm, equivalent to microsphere;^ Ziegler's ecto- spJiere and entosphere, applied to the cortical and medullary zones respectively ; and Meves's idio::^ome, applied to the "attraction-sphere " of the spermatids.^ This profusion of technical terms has arisen through the desire to avoid ambiguity in the use of the term " attrac- tion-sphere," which, like the word " Nebenkern " (p. 163), has been applied to bodies of quite different origin and fate. If we adhere to Van Beneden's original use of the term it must be confined to the body surrounding the centrosome, forming a part of, or directly derived from, an aster, and giving rise wholly or in part to the succeeding aster. Meves ('96), Rawitz ('96), Erlanger ('97, 2), and others have, however, clearly shown that the " attraction-sphere " surrounding the centro- some (in testis-cells) may not only contain other material derived from the cytoplasm, e.g. the " centrodeutoplasm " of Erlanger, but may take no direct part in the succeeding aster-formation, disintegrating and scattering through the cell as the new aster forms (Fig. 2^). In

Other cases a sphere closely simulating an attraction-sphere may arise in the cytoplasm without apparent relation to the centrosomes or to the preceding aster, e.g. the yolk-nucleus or the sphere from which the acrosome arises in mammalian spermatogenesis.^ To call such structures ''attraction-spheres" or " archoplasm-masses " is to beg an important question; and in all such doubtful cases the simple word sphere should be used.^ In case of the aster itself we may, for descriptive purposes, employ Strasburger's convenient and non-com- mittal term centrosphere, to designate in a somewhat vague and general way the central mass of the aster surrounding the centrosome, leavinc^ its exact relation to Van Beneden's attraction-sphere to be determined in each individual case. Where the centrosphere shows two concen- tric zones (medullary and cortical), they may be well designated with Ziegler as entosphere (" centrosome" of Boveri) and ectosp/iere.

As regards the structure of the centrosphere, two well-marked types have been described. In one of these, described by Van Beneden in Ascaris, by Heidenhain in leucocytes, by Driiner and Braus in divid- ing cells of Amphibia, and by Francotte, Van der Stricht, Lillie, Kos- tanecki, and others, in various segmenting eggs, the centrosphere has a radiate structure, being traversed by rays which stretch between the centrosome and the peripheral microsome-circle (Fig. 152, D, E, F\ when the latter exists. In the other form, described by Vejdovsky in the eggs of Rhynchelmis, by Solger and Zimmermann in pigment-cells, by myself in Nereis, by Riickert in Cyclops, by Mead in CJicctoptcnts, Griffin in Thalassejua, Coe in Cerebratulus, Gardiner in Polycliccrus, and many others, the centrosphere has a non-radial reticular or vesicu- lar structure, in which the centrosomes lie (Figs. 152, //, 155). Kos- tanecki and others have endeavoured to show that such structures are artifacts, insisting that in perfectly fixed material the astral rays always traverse the centrosphere to the centrosome. This interpretation is, however, contradicted by the fact that the new asters developing in the centrospheres during the anaphases and telophases of such forms as Thalassemia or Cerebratulus (Figs. 99, 155) show perfect fixation of the rays. The reticular centrosphere almost certainly arises as a nor- mal differentiation of the interior of the aster, which, as Griffin ('q6) has suggested, probably marks the beginning of the degeneration of the whole astral apparatus, to make way for the newly developing system.

The radial centrosphere is in Ascaris divided into cortical and medul- lary zones, as already described (p. 310), the aster being bounded by a distinct circle of microsomes. The true interpretation of these zones was given through Heidenhain's beautiful studies on the asters in leu- cocytes, and the still more thorough later work of Driiner on the sper- matocyte-divisions of the salamander. In leucocytes (Fig. 49) the large persistent aster has at its centre a well-marked radial sphere bounded by a circle of microsomes, as described by Van Beneden, but without division into cortical and medullary zones. The astral rays,

however, show indications of other circles of microsomes lying outside the centrosphere. Druner found that a whole series of such concen- tric circles might exist (in the cell shown in Fig. 1 56 no less than nine), but that the innermost two are often especially distinct, so as to mark off a centrosphere composed of a medul- lary and a cortical zone precisely as described by Van Beneden. These observations show conclusively that the centrosphere of the radial type is merely the innermost portion of the aster, which acquires a boundary through the especial development of a ring of microsomes, or other- wise, and which often further

The nucleus lies below. Above is the enor- ity SO aS tO appear like a CCUtrOSOme mous aster, the centrosome at its centre, its ^^ ^ ^ ,>^_ j^^ ThysanOZOOIl (Van der Stricht) only a single ring of micro- somes exists, and this lies at the boundary between the medullary and cortical zones (Fig. 152, D), the latter differing from the outer region only in the greater delicacy of the rays and their lack of staining-capacity, thus producing a '* Heller Hof." In other cases, no " microsome-circles " exist; but even here a clear zone often surrounds the centrosome {e.g. in Physa, t. Kostanecki and Wierzejski), like that seen in the cortical zone of TJiysanozoon.

There are some observations indicating that the entosphere (medul- lary zone) may be directly derived from the centrosome (central granule). This is the conclusion reached by Lillie in the case of Unio referred to above, where, during the prophases of the second polar spindle, the central granule enlarges and breaks up into a group of granules from which the new entosphere is formed. Van der Stricht ('98) reaches a similar conclusion in case of the first polar spindle of TJiysanozoon. We may perhaps give the same interpretation to the large pluricorpuscular centrum of echinoderms (p. 314). This obser- vation may be used in support of the probability that the astral rays

rays showing indications of nine concentric circles of microsomes. The area within the second circle probably represents the " attrac- tion-sphere " of Van Beneden. may be actually derived from the centrosome (p. 321) ; but Lillie finds in some cases that in the same mitosis the entosphere is formed by a different process, arising by a differentiation of the cytoplasm around the central granule. The former case, therefore, may be interpreted to mean simply that the centrosome may give rise to other cytoplasmic elements (as has already been shown in the formation of the sperma- tozoon, p. 172), the material of which may then contribute either directly or indirectly to the building of the aster ; and the facts do not come into collision with the view that the astral rays are in f^en- eral formed from the cytoplasmic substance.

A minute analysis of the various parts of the cell leads to the con- clusion that all cell-organs, whether temporary or ** permanent," are local differentiations of a common structural basis. Temporary organs, such as cilia or pseudopodia, are formed out of this basis, persist lor a time, and finally merge their identity in the common basis again. Per- manent organs, such as the nucleus or plastids, are constant areas in the same basis, which never are formed de novo, but arise by the divi- sion of preexisting areas of the same kind. These two extremes are, however, connected by various intermediate gradations, examples of which are the contractile vacuoles of Protozoa, which belong to the category of temporary organs, yet in many cases are handed on from one cell to another by fission, and the attraction-spheres and asters, which may either persist from cell to cell or disappear and re-form about the centrosome. There is now considerable evidence that the centrosome itself may in some cases have the character of a perma- nent organ, in others may disappear and re-form like the asters.

The facts point toward the conclusion, which has been especially urged by De Vries and Wiesner, that the power of division, not only of the cell-organs, but also of the cell as a whole, may have its root in a like power on the part of more elementary masses or units of which the structural basis is itself built, tJie degree of permaiieuee in the eell- organs depending on the degree of cohesion manifested by these elemen- tary bodies. If such bodies exist, they must, however, in their primary form, lie beyond the present Hmits of the microscope, the visible struc- tures arising by their enlargement or aggregation. The cell, therefore, cannot be regarded as a colony of ''granules " or other gross morpho- logical elements. The phenomena of cell-division show, however, that the dividing substance tends to differentiate itself into several orders of visible morphological aggregates, as is most clearly shown in the nuclear substance. Here the highest term is the plurivalent chromo- some, the lowest the smallest visible dividing basichromatin-grains,

while the intermediate terms are formed by the successive aggrega- tion of these to form the chromatin-granules of which the dividing chromosomes consist. Whether any or all of these bodies are *' indi- viduals " is a question of words. The facts point, however, to the conclusion that at the bottom of the series there must be masses that cannot be further split up without loss of their characteristic proper- ties, and which form the elementary morphological units of the nucleus. In case of the cytoplasm the evidence is far less satisfactory. Could Rabl's theory of fibrillar persistence, as developed by Heiden- hain and Kostanecki, be established, we should indeed have almost a demonstration of panmeristic division in the cytoplasm. At present, however, the facts do not admit the acceptance of that theory, and the division of the visible cytoplasmic granules must remain a quite open question. Yet w^e should remember that the dividing plastids of plant-cells are often very minute, and that in the centrosome we have a body, no larger in many cases than a ** microsome," which is positively known to be in some cases a persistent morphological ele- ment, having the power of growth, division, and persistence in the daughter-cells. Probably these powers of the centrosome would never have been discovered were it not that its staining-capacity ren- ders it conspicuous and its position at the focus of the astral rays isolates it for observation. When we consider the analogy between the centrosome and the basichromatin-grains, when we recall the evidence that the latter graduate into the oxychromatin-granules, and these in turn into the cytomicrosomes, we must admit that Brlicke's cautious suggestion that the whole cell might be a congeries of self- propagating units of a lower order is sufficiently supported by fact to constitute a legitimate working hypothesis.

Van Beneden and Julin. — La segmentation chez les Ascidiens et ses rapports avec Briicke, C. — Die Elementarorganismen : lVie?ier Silz.-Ber., XIAV. 1861. Biitschli, 0. — Protoplasma. (See List I.) Delage. Yves. — La structure du protoplasma, et les theories sur Theredite. Parts, Hacker. V. — Uber den heutigen Stand der Centrosomenfrage : Vcrh. d. deutsch. Zo'dl. Ges. 1894. Heidenhain, M. — (See List I.) Herla. V. — Etude des variations de la mitose chez Pascaride megalocephale : Arch.