114 CYTOKINESIS. 



sion is prearranged in the cytoplasm and that in normal cell division the mitotic 

 figure is oriented by stresses and movements within the cytoplasm which bring the 

 spindle to rest with its equator in the plane of cleavage. The equality or inequal- 

 ity of cell division is therefore a function of the cytoplasm. 



{d). Qtt-ality of Daughter Cells. — Finally we consider homogenous and hete- 

 rogenous divisions or the differential distribution of different substances to daughter 

 cells. In the case of both nucleus and centrosome there is every evidence that the 

 most exact halving and distribution of their substance, not only quantitatively but 

 qualitatively as well, occurs at every division. So fixr as my observations on the 

 gasteropods go -there is absolutely no evidence that centrosomes or chromosomes 

 undergo the slightest qualitative changes as development advances. 



With the cytojjlasm, however, the case is quite different; there is here not only 

 differential distribution of yolk but also of sphere substance and of different kinds of 

 cytoplasm [viz. granular or hyaline.) Thus the yolk is entii'ely contained in the 

 macromeres while the micromeres are wholly free from it. The sphere substance 

 too, after the first two cleavages, is differential!}^ distributed at every division, 

 always passing into that daughter cell which lies nearer the animal pole. Here it 

 slowly disintegrates and disappears and in its place a clear hyaline kind of plasma 

 is formed (see text figs. XIX-XXV). If the sphere substance, or the plasma into 

 which it is transformed, maintains the same kind of polarity after its transformation 

 that it had before, there would result an aggregation of this substance or plasma in 

 the cells lying near the animal pole. The result of this differential distribution of 

 the sphere substance may be summarized as follows : — The first quartette contains 

 two and one-half generations of sphere substance, i. e., all the sphere substance of 

 the first and second cleavages and one-half that of the third ; the second, third and 

 fourth quartettes each contain one generation of sphere substance, i. e., one-half 

 that of the division by which they were formed and one-half that of the preceding 

 division. The macromeres never contain more than one-half generation of sphere 

 substance. 



Finally, in the subdivisions of the quartettes, the cells l^'ing nearest the animal 

 pole receive most of the sphere substance or of the plasma to which it gives rise. 

 Since the sphere substance varies in quantity in different cells, being always pro- 

 portional to the size of the cell, the distribution of the substance by generations 

 does not give nxij idea of its quantitative distribution. However, the first quartette 

 not only receives a larger number of generations of the sphere substance than any 

 other but also a larger quantity of this substance. Associated with this maj^ be the 

 fact that the cytoplasm of the first quartette is always clearer and less granular tlian 

 that of the second and third. 



The sphere substance is formed of hyaloplasm from the cell body and achroma- 

 tin from the nucleus and the differential distribution of this substance may be an im- 

 l^ortant factor of differentiation. If the nucleus controls the cell as DeVries, Weis- 

 mann and Roux maintain,^ we have in this differential distribution of the spheres a 

 possible mechanism for such control, as well as for differentitation. However, the 



' See p. .52. 



b 



