CHAPTEE XXXIV. 

 General Considerations on the Mollusca-* 



In attempting to combine into one general scheme the de- 

 velopmental phenomena described in the preceding chapters, in 

 connection with the different divisions of the Mollusca, we cannot, of 

 course, take into account all the very different ontogenetic processes, 

 but can only select from among them those that are of the more 

 general significance. 



Even in the cleavage of the egg, great variety prevails. Thus we 

 rind, on the one hand, that the meroblastic type of cleavage attains- 

 its highest development among the Mollusca (Cephalopoda) but, on 

 the other, that total and, at first equal, but soon after unequal 

 cleavage is still more common in eggs of this phylum (Chiton, 

 Gastropoda), and so also is a type of cleavage which from the first is 

 unequal (Solenogastres, Lamellibranchia, Solenoconchae). The cause 

 of this difference is to be sought in the varying amount of yolk 

 contained within the eggs but, even among eggs showing total 

 cleavage, there are some that, through a secondary separation of the 

 volk-mass from the blastomeres, lead us to the meroblastic method 

 of cleavage (Nassa). The latter is to he explained as due to the 

 extraordinarily large amount of yolk in the egg, and this same 

 peculiaritv further determines so great a modification of the early 

 ontogenetic phenomena in the Cephalopoda, that this division must 

 be left almost entirely out of account in our comparative review. 

 Within each separate division of the Mollusca, the phenomena of 

 cleavage are very similar and very regular. 



The formation of the primary germ-layers takes place through the 

 invagination of a blastula with a more or less wide cleavage-cavity, 

 or, where the latter is wanting, and the egg is richer in yolk, through 

 epibole. These two processes are found in nearly related forms, or 



* See note, p. 1. 



