32 ART. 4. N. YATSU : 



folded double along one of its diameters, and is secondarily 

 divided into two valves along the posterior edge. In the furrow 

 between the two posterior mantle folds the shell is at first secreted 

 just as in other regions, but it is soon separated from the epi- 

 thelium of the mantle furrow (PI. IV., Fig. 64.). This is ap- 

 parently due to the rapid growth of the posterior mantle-folds. 

 The cuticula thus separated must retain its original thickness, as 

 no new secretion can be added. But in the oldest embryos I was 

 able to rear from the eggs the division into two valves had not 

 yet taken place (PI. IV., Fig. 70.). 



VII. EECAPITULATION. 



Let me recapitulate the principal developmental process which 

 took place during the growth of eggs : — 



1). The eggs and sperm are discharged early in the morn- 

 ing. The fertilization thus takes place outside the body. 



2). The egg is holoblastic, the cleavage being total and equal. 



3). The blastomeres are so arranged at the 32-cell stage as 

 to form two layers, each consisting of 16 blastomeres as in the 

 Phylactolœmata. 



4). The cœloblastula is formed. 



5). The gastrula is next formed. 



6). The blastopore is closed. 



7). The archenteron persists as the cavity of the alimentary 

 canal. 



8). The entoblast gives rise to the posterior half of the 

 oesophagus and to the stomach. 



9). The mesoblast arises from the entoblastic walls, and has 

 two different destinations : — 



