94 



BIOLOGICAL LECTURES. 



that cleavage has a phylogenetic significance. Others regard 

 these resemblances as superficial and accidental, and cleavage 

 as having no phylogenetic significance whatever, except as a 

 possible indication of the mode of origin of the primitive 

 metazoon from its protozoon ancestor. 



It is noteworthy in this connection that the theory which one 

 adopts is apt to be dependent upon the animal which he has 

 studied. In some groups, for example, in many annelids, mol- 

 luscs, platodes, ascidians, and nematodes, the cleavage always 

 follows a definite form or pattern, and, apparently, is so corre- 

 lated with differentiation that Conklin has proposed to designate 

 it as determinate, as opposed to the indeterminate type found 

 in many vertebrates, echinoderms, and Cnidaria, where the 

 cleavage seems to be very indefinite and inconstant. Investi- 

 gators who have worked on the former groups are generally 

 inclined to give greater importance to cleavage than those who 

 have studied the latter. No one who has followed the cell 

 lineage of an annelid or a mollusc can deny an apparent differ- 

 entiation, manifest even in the earliest cleavages, and the notion 

 of such differentiation seems not disproved by the results of 

 the experimental embryologists. The argument in this line has 

 been so ably presented in another place ^ that I will not attempt 

 to repeat it here, but pass to the more especial discussion of 

 equal and ungqual cleavages. 



It is a fact, familiar to all who have ever seen a segmenting 

 ovum, that the first cleavage in all holoblastic eggs divides the 

 cell into two equal or unequal parts, according to the particular 

 animal studied, the widest possible variations appearing among 

 closely related animals. Inasmuch as unequal cleavage is usu- 

 ally associated with the presence of a considerable amount of 

 food yolk, it was formerly supposed that yolk alone is respon- 

 sible for this form of division. A careful study of the develop- 

 ing ovum shows, however, that this explanation will not apply 

 in all, or even in the majority of cases, and I shall offer some 

 suggestions, based on a study of the annelids, which may, 

 perhaps, throw some light on the subject. For this purpose 



1 Conklin, "Embryology of Crepidula," Biol. Lectures (1896, 1S97), and/(wr«. 

 of Morph., vol. xiii. 



