442 HOLMES. [Vol. XVI . 



to extrinsic mechanical conditions. The resemblances between 

 the cleavage of mollusks and annelids are too numerous and 

 too close to be explained in this manner, or even, as I believe, 

 by the principle of " parallel precocious segregation," But 

 while similarity of cleavage in different groups, provided it is 

 long continued and close, may be held to indicate genetic affin- 

 ity, the converse of the proposition, that differences in the form 

 of cleavage imply lack of relationship, does not always follow. 

 It is well known that the form of cleavage may vary even in 

 eggs of the same species. The summer and winter eggs of 

 daphnids have entirely different forms of cleavage, yet both 

 develop into the same kind of embryos. In many vertebrates 

 the particular manner in which the Qgg is divided appears to be 

 a matter of little moment as regards its future development. 

 In Renilla, Wilson found that the early cleavage of the egg 

 presented great variations which were without any apparent 

 influence on the end result. 



It is obvious that no general rule can be drawn regarding the 

 phyletic significance of cleavage. In some groups cleavage has, 

 doubtless, a high degree of "systematic worth"; in others it 

 may have very little. Similarly, as a mark of affinity between 

 the different groups of animals, as in the case of gasteropods 

 and annelids, cleavage may be a character of considerable value ; 

 or again, as in the case of the gasteropods and cephalopods, its 

 evidence may be of little weight. It seems probable that simi- 

 larities of cleavage should be regarded as an incidental and not 

 a necessary expression of genetic affinity. Whether or not the 

 relationship between different classes of animals expresses itself 

 in the early cleavage of the ovum may depend largely upon 

 external conditions, or upon the amount of yolk in the egg, or, 

 perhaps, upon the degree of cytoplasmic differentiation that 

 has been reached before cleavage begins. It is not my pur- 

 pose, however, to attempt to discuss what may be the reason 

 for the varying morphological significance of cleavage forms in 

 different groups of animals. The fact I would emphasize is 

 that mere cell genealogy stands in no necessary relation to the 

 genealogy of organs. This conclusion, which is supported by 

 a variety of considerations, receives a strong confirmation — if 



