Mitotic spindle

“ The growth of the chromatin is an indispensable prelude to its reproduction (Boveri’s Law of Proportional Growth) . The chromatin is therefore incapable of undergoing reproductive fission in two successive mitotic divisions when these are not separated by a resting (i.e. growth) period. In addition to this, the “bipolar” condition of the adult chromosomes, which determines its mode of attachment to mantle fibres from both poles of the spindle, is not possessed by the unripe chromatin. ”

“ The reorganizing daughter nuclei, in the only section secured in this stage, are too densely stained to show their structure well; they appear simply to be densely granular. The spindle fibers show very plainly, but do not as yet show any thickenings in the center preparatory to the formation of a cell membrane. A most careful search has failed to reveal any suggestions of a centrosphere or centrosome at the apex of the spindle where one might be expected to occur. ”

“ In reports upon Pygæra and Sphinx he changed the name nebenkern to mitosome. The centrosome lies in front of the nucleus and forms the point of the head. This he calls the nebenkern. From the spindle remains arise two bodies. A large, fibrous one from the equatorial end has a clear space around it and the axial filament passes through it. It soon elongates and forms the tail. This is the large mitosome. The other is much smaller and arises from the polar end of the spindle fibers. It takes its place in the angle between the large mitosome and the nucleus. ”

“ The spindle does not occupy the entire nucleus, and at each side the protoplasin retains the reticular form similar to its structure before the spindle began to form. The most noteworthy variation in structure occurring in the cell at this stage of the division is in the blepharoplast, which has undergone a striking change since the last stage. ”

“ A very obvious theory therefore suggests itself:—that while the formation of the second polar body is purely a phenomenon of maturation in most animal eggs, and is independent of fertilization,—in the eggs of a number of other animals, on the other hand, and especially among Arthropods, the formation of the second nuclear spindle is the result of a stimulus due to the entrance of a spermatozoon. ”

“ During the development of the egg the formation of young cells, by division of the segmentation nucleus, is so rapid and continuous that the achromatic figure, with the centrosome in the pole of the spindle, is a readily recognizable object in each cell. The polar and spindle-like radiations are in evidence during karyokinesis, and have apparently a temporary endurance and function. ”

“ The spindle fibers run into this mass and end rather abruptly, not extending up into it. The spindle fibers can not be confounded with the fibers of the reticulum. There can be no doubt that no fibers run from the spindle to the blepharoplast in any sense in which the spindle fibers focus on a centrosome or centro- sphere when such an organ is present. The spindle formation and structure were not investigated in much detail by Ikeno and Hi rase ”

“ There were no dividing spermatogonia and few spermatocyte mitoses. Most of the spermatocytes contained 10 chromosomes, one of which was plainly an unequal pair. In a few testes the number was 11, indicating that pupæ of two species had been collected. Figure 187 shows the metaphase of first spermatocyte mitosis with the unequal pair in metakinesis. Figures 188 and 189 are first spermatocyte equatorial plates of the two species, containing 10 and 11 chromosomes respectively. Figure 190 is a first spermatocyte spindle in anaphase, showing the unequal pair behind the other chromosomes. ”

“ The spindle fibers have bulged out on each side of the old mother-cell nucleus, reaching the cell wall on each side. They have contracted away from the daughter nuclei, and seem at their outer ends to be in the process of gradual transformation into the normal reticulum of the cytoplasm. No visible thickening occurs on the fibers where the new plasma membrane is to be formed, but a space free of granules and stainable matter clearly shows where the new plasma membrane delimiting the two cells will form. This is particularly well shown in a photomicrograph of the cell shown in figure 63. ”

“ On the contractile theory, the radiations which form the body of the spindle, either by actual traction of the supposed fibrillse or by their pressure on the nucleus which they surround, might impel during karyokinesis the dividing chromosome elements toward the poles of the spindle, to form there the daughter nuclei. On the dynamical theory, the chemical and physical energy in the centrosome might influence the cell plasm and the nucleus and attract the chromosome elements of the nucleus to the poles of the spindle. ”

“ One of the most typical cases of centrosomes in plants is that described by Swingle in 1897 as occurring in Stypocaulon (114) . Here a minute, deeph' staining, dumb-bell-shaped body occurs at the pole of the spindle in karyokinesis, which at the close of division divides into two. Both of these remain in close connection with the nuclear membrane, but travel in opposite directions until they come to lie at opposite points on the equator of the nucleus. ”

“ During the metaphase, or slightly before or after, the nuclear membrane breaks up and would appear to become transformed into spindle fibers, which remain in the position previously occupied by the nuclear membrane and preserve the outline of the nucleus.
In an early anaphase of division (figs. 33 and 62) the dividing nucleus presents a perfectly normal appearance so far as the main features of division are concerned. The chromosomes have just pulled apart and are approaching their respective poles ”

“ Fig. 8.—Preparation for Mitosis. a, Nucleus of “1/2 blastomere” of Ascaris megalocephala bivalens in resting condition; b and c, nuclei from sister 1/2 blastomeres in preparation for mitosis.
In the same worm (var. univalens) Boveri (1888 and 1899) found that occasional abnormalities in maturation resulted in the suppression of the first polar body and the inclusion of its chromosomes in the second maturation spindle; the egg-nucleus at the time of fertilization thus having two chromosomes instead of one, while the spermatozoon nucleus has only one. ”

“ In connection with these insect forms it is of interest to find in the spermatogenesis of Sagitta a body which stains like chromatin and behaves somewhat like the accessory chromosome. It is found in all resting stages of the spermatogonia, closely applied to the nuclear membrane (fig. 226) . It divides before each spermatogonial mitosis (fig. 227) and, though not often discernible in the spindle, appears in the next generation. Figure 228 is the last spermatogonial mitosis, and figure 229 shows the element x, and the chromosomes paired at one pole of the spindle. ”

“ The cells of the higher plants do not possess such individualised centres; they have probably disappeared in the course of phylogenetic development: in spite of this, however, in the nuclear division-figures the fibrillae of the cell-plasma are seen to radiate from two opposite poles. In both animal and plant cells a fibrillar bipolar spindle is formed, the fibrillae of which grasp the longitudinally divided chromosomes from two opposite sides and arrange them on the equatorial plane of the spindle as the so-called nuclear or equatorial plate. ”

“ There are, however, serious objections to the interpretation of mitosis as an adaptation to ensure this equal distribution of the chromatin. Not only does the occurrence of amitosis show that the mitotic mechanism is not essential for either nuclear or cytoplasmic division, but direct division may occur31 in the life-history of the germ cells, the very point at which it should not occur had mitosis the significance usually attached to it. ”

“ In his succeeding paper (’86, 1) he studied the “nebenkern” spindle remains—in the spermatogonia and spermatocytes of pulmonates. In the spermatids, as in the former generations of cells, the nebenkern grows out of the nucleus, where it, with the chromatin, had formed the spireme. It appears as a loop, which becomes larger, twisted, and entangled, and finally breaks loose from the nucleus. Later it goes to form the spiral covering of the primary tail, changing it to the axial filament and true tail. ”

“ Modern researches have shown that the nucleus, when the cell divides, assumes the form of a spindle of fibres, associated with which are distinct bodies called chromosomes, that the number of these chromosomes where it can be counted is constant for all individuals of the same species, and that before the gametes are ready for fertilisation two cell-divisions take place, which result in the reduction of the number of chromosomes to half the original number. ”

“ In Dictyota, also, Mottier (89 and 90) has demonstrated the occurrence of centrosomes similar to those described in Stypocauhm by Swingle, and in Fucm by Strasburger. They are small, deeply staining bodies, located in the center of a large aster, and are apparenth' permanent organs of the cell, reproducing by division during the reconstruction of the daughter nucleus. The centrosomes here also are intimately connected with spindle formation, as in the case of Stypocautlsm, the fibers growing into the nucleus from each centrosome in forming the spindle. ”

“ For the larger pieces, the spindles should be made very strong and exact. The instructions on this head are, that for a piece of nine mutations, let the spindle be, at the large end, one inch in diameter, and continue that thickness as far as the seventh mutation, and from thence to the fifth, let its diameter be three-quarters of an inch. The other proportions, then, are, from the fourth to the second, half an inch ”

“ They are always surrounded by ra r s of kinoplasm, which become very abundant during spindle formation and division and are not surrounded by any differentiated sphere of any sort, as so commonly occurs in the centrosomes of animals. In spindle formation the centrosome appears to be of prime importance, a bundle of fibers starting from each centrosome and gradually extending into the nucleus until the spindle is completely formed. Swingle's studies were made with growing vegetative tips, which would indicate that the centrosome here is probably a permanent organ in all stages of growth. ”

“ In the metaphase of all spermatogonial mitoses where it was possible to count accurately, 20 chromosomes were found, 19 large ones of approximately equal size, and 1 small spherical one (figs. 169, 170) . There is nothing in the resting nucleus of the spermatogonia which would suggest either a nucleolus or an accessory chromosome. The chromatin stains well during the whole growth period of the spermatocytes, but it is impossible to separate the period into so definite stages as in most other forms. ”

“ It will be objected that neither the beginning nor the end of embryonic development can possibly depend upon the quantity of nucleoplasm in the segmentation nucleus, since the amount may be continually increased by growth; for it is well known that during embryonic development the nuclear substance increases with astonishing rapidity. ”