CLEAVAGE AND DIFFERENTIATION. 41 



Numberless instances are known in which homologous adult 

 parts arise in different ways in closely related animals — e.g., the 

 central nervous system of teleosts and of selachians, the noto- 

 chord and mesoblastic somites of amphioxus and of other verte- 

 brates, the body musculature of Lopadorynchus and of other 

 annelids, etc. — and yet who holds on this account that there are 

 no homologies whatsoever between any embryonic parts ? The 

 objections to such homologies are objections only to the view 

 that they are complete and universal ; among certain phyla and 

 recognizing certain modifications, even the germ layers are 

 homologous, and within perhaps even narrower limits there is 

 homology of blastomeres. How else is it possible to explain 

 the remarkable resemblances which have been pointed out 

 between the annelids and mollusks, resemblances which are 

 inherited with such tenacity as to be found throughout all the 

 species, genera, and orders of an entire phylum ? The fact that 

 blastomeres are not universally homologous should not cause 

 us to shut our eyes to certain striking homologies which do 

 exist. Certainly within the limits here indicated the existence 

 of cell homologies seems extremely probable, and their impor- 

 tance will not be overlooked save by those who are concerned 

 only with " universal laws." 



If such resemblances between blastomeres are homologies, 

 what follows .'' (i) Cleavage has a certain phylogenetic signifi- 

 cance, and, although possibly more liable to modifications than 

 larval or adult stages and hence less trustworthy as a test of 

 homology and of genetic relationship, it may in certain cases at 

 least preserve ancestral conditions even after they have dis- 

 appeared in end stages (annelids and mollusks). Incidentally, 

 the homologies of cleavage added to those of embryonic and 

 larval structures indicate the close relationship of annelids 

 and mollusks, whereas the entire embryological history only 

 serves to widen the gap between the cephalopods and other 

 mollusks, 



(2) The early cleavages are morphologically more important 

 than later ones. This follows from the notion of determinate 

 cleavage, some of the earlier blastomeres being destined to 

 form entire regions or organs of the animal, but principally 



