This conclusion received a very important support in the work of vom Rath on amitosis in the testis ('93). On the basis of a comparative study of amitosis in the testis-cells of vertebrates, mollusks, and arthropods he concludes that amitosis never occurs in the spermproducing cells (spermatogonia, etc.), but only in the supporting cells (Randzellen, Stutzzellen). The former multiply through mitosis alone. The two kinds of cells have, it is true, a common origin in cells which divide mitotically. When, however, they have once become differentiated, they remain absolutely distinct ; amitosis never takes place in the series which finally results in the formation of spermatozoa, and the amitotically dividing " supporting-cells " sooner or later perish. Vom Rath thus reached the remarkable conclusion that " when once a cell has undergone amitotic division it has received its death-warrant ; it may indeed continue for a time to divide by amitosis, but inevitably perishes in the end." ('91, p. 331.) Whether this conclusion can be accepted without modification remains to be seen. Flemming himself regards it as too extreme, and is inclined to accept Meves' conclusion that amitosis may occur in the sperm-producing cells of the testis. The same conclusion is reached by Preusse in the case of insect-ovaries. There can be no doubt, however, that Flemming's hypothesis in a general way represents the truth, and that in the vast majority of cases amitosis is a secondary process which does not fall in the generative series of cell-divisions.
Three distinct elements are involved in the typical mode of celldivision by mitosis; namely, the centrosome, the chromosome, and the cell-body. Of these, the centrosome may be considered the organ of division par excellence; for as a rule it leads the way in division, and under its influence, in some unknown manner, is organized the astral system which is the immediate instrument of division. This system appears in the form of two asters, each containing one of the daughter-centrosomes and connected by a spindle to form an amphiaster. It arises as a differentiation or morphological rearrangement of the general cell-reticulum, the asters being formed from the extranuclear reticulum, the spindle sometimes from the linin-network, sometimes from the cyto-reticulum, sometimes from both.
The chromosomes, always of the same number in a given species (with only apparent exceptions), arise by the transformation of the chromatin-reticulum into a thread which breaks into segments and splits lengthwise throughout its whole extent. The two halves are thereupon transported in opposite directions along the spindle to its respective poles and there enter into the formation of the two corresponding daughter-nuclei. The spireme-thread, and hence the
chromosome, is formed as a single series of chromatin-granules or chromomeres which, by their fission, cause the splitting of the thread. Every individual chromatin-granule therefore contributes its quota to each of the daughter-nuclei.
The mechanism of mitosis is imperfectly understood. There is good reason to believe that the fission of the chromatin-granules, and hence the splitting of the thread, is not caused by division of the centrosome, but only accompanies it as a parallel phenomenon. The divergence of the daughter-chromosomes, on the other hand, is in some manner determined by the spindle-fibres developed under the influence of the centrosomes. There are cogent reasons for the view that some at least of these fibres are contractile elements which, like muscle-fibres, drag the daughter-chromosomes asunder ; while other spindle-fibres act as supporting and guiding elements, and probably by their elongation push the spindle-poles apart. The contractility hypothesis is, however, difficult to apply in certain cases, and is probably an incomplete explanation which awaits further investigation. The functions of the astral rays are involved in even greater doubt, being regarded by some 'investigators as contractile elements like those of the spindle, by others as rigid supporting fibres like those of the central spindle. In either case one of their functions is probably to hold the kinetic centre in a fixed position while the chromosomes are pulled apart. Whether they play any part in division of the cell-body is unknown ; but it must be remembered that the size of the aster is directly related to that of the resulting cell (p. 5 I) a fact which indicates a very intimate relation between the aster and the dividing cell-body. On the other hand, in amitosis the cell-body may divide in the absence of asters.
These facts show that mitosis is due to the co-ordinate play of an extremely complex system Of forces which are as yet scarcely comprehended. Its purpose is, however, as obvious as its physiological explanation is difficult. It is the end of mitosis to divide every part of the chromatin of the mother-cell equally between the daughter-nuclei. All the other operations are tributary to this. We may therefore regard the mitotic figure as essentially an apparatus for the distribution of the hereditary substance, and in this sense as the especial instrument of inheritance.