CLEAVAGE AND DIFFERENTIATION. 39 



The cause of such resemblances, like the cause of determi- 

 nate cleavage and of the constancy of specific characters, must 

 be found in protoplasmic structure, and I cannot escape the 

 conviction that these likenesses belong to the same category 

 with the fundamental resemblances between gastrulae, larvae, 

 and adults. Whatever criterion of homology one may adopt 

 — whether similarity of origin, position, history, or destiny, or 

 all of these combined — certain of these resemblances in 

 cleavage bear all the marks of true homologies. 



It is freely granted once for all that even in the limited form 

 in which it is here maintained there are serious difficulties in 

 the way of the doctrine of cell homology. The most important 

 of these difficulties are the following: (i) Related animals do 

 not always have similar cleavage, e.g., cephalopods and other 

 mollusks ; triclades, and polyclades. Even within a single order 

 there may be important differences; thus the cleavage is 

 markedly radial in Discocoelis and as markedly bilateral in 

 Polychaerus. Among the Crustacea there are four types of 

 cleavage (see Korschelt und H eider, Lehrbuch der Entivick- 

 hingsgeschichte) : (a) total and equal, {b) total and later super- 

 ficial, (^) purely superficial, (a') discoidal. Finally, contradictions 

 reach a climax among the Daphnidae, where the summer and 

 winter eggs of the same species may belong to wholly different 

 types of cleavage, as Watase ^ has pointed out. No cell homol- 

 ogy is recognizable in such cases, and possibly none exists. (2) 

 Similar larval or adult parts may arise through very different 

 types of cleavage; e.g., the primitive streak of sauropsida and 

 mammalia, the adult structures of amphioxus as compared with 

 most other vertebrates, the shell gland of gasteropods and 

 cephalopods. Such cases show that adult homologies are not 

 necessarily dependent upon cell homologies. (3) Similarities 

 in cleavage may not lead to similarities in subsequent stages, 

 e.g., the cleavage of certain polyclades is closely like that of 

 annelids and mollusks, and yet the cells which are mesomeres 

 in one case are ectomeres in the other. However, the discov- 

 ery of larval mesenchyme in Unio and Crepidula has lessened 

 the difference in this regard, and it is possible that a further 



1 Watase, S., " Stvidies on Ce-p\\?i\o\)ods" /oin>ial of Morphology, IV, 1891. 



