No. I.] THE EMBRYOLOGY OF THE UNION/DAE. 45 



point out the way for future investigation, and give us good 

 cause for hope that, when we know more thoroughly the phe- 

 nomena which accompany differentiation, we shall not be so 

 much in the dark as to its determining factors. 



For many years unequal cleavage has been explained as due 

 to the arrangement of yolk within the dividing cell. So long 

 as it was considered unnecessary to determine the prospective 

 value of individual blastomeres, hardly any other explanation 

 of unequal cell-division in ontogeny was possible. If the 

 cleavage of an alecithal ovum was unequal from the start, 

 e.g., the lamellibranch ovum, a satisfactory explanation was 

 ready to hand : it was due to the inherited effects of the 

 lost food yolk. If unequal divisions occurred at any stage of 

 development, the presence of yolk was sufficient explanation. 

 Similarly yolk was held to retard cell-division by hampering 

 the free action of the cytoplasm. It has even been held that 

 the rapidity of cell-division is proportional to the concentration 

 of the protoplasm. It is of course true in many instances, that 

 unequal cleavage is due to the presence of yolk, but this is, 

 nevertheless, only one phase of a more general law : — Unequal 

 cleavage is conditioned by the constitution of the segmenting 

 ovum, and always means the precocious localization of an organ 

 or set of organs in the larger cell. This organ may be the 

 entoderm, in which case it is usually accompanied by yolk ; 

 but the inequality of the first two cells in the annelids and 

 molluscs is the earliest visible indication of another differenti- 

 ation, the larger cell containing the two somatoblasts. The 

 more precocious the differentiation of the organs of the somato- 

 blasts, the greater the difference in the size of the cells {cf. 

 Unio). The two cells may be equal in size when the organs in 

 question are not precociously developed. The same principles 

 suffice to explain unequal divisions throughout the cytogeny. 

 This of course traces back unequal cleavage to protoplasmic 

 structure and is in agreement with Watase, who says (No. 60, 

 p. 294) : " The cause of unequal cleavage in the various cases 

 which we have examined appears to me to be an internal one 

 due to the peculiarities of the particular protoplasmic structure 

 which composes the segment or segments." 



