358 L. DONGASTER. 



dorsal wall of the archenteroii took some share in tlie back- 

 ward growtli, but sections lend no support to this assumption. 

 "J'he folds grow back to the posterior end of the arclicnteron, 

 •separating off the coeloinic cavities completely, and the folds 

 become pressed together so as to form a thin longitiidiual 

 septum running through the animal. As the embryo elongates 

 its cavities become reduced, and this is further increased by 

 the structure of the somatic mesoderm, the inner boundaries 

 of which become irregular, and cracks appear between the 

 cells, so that the layer becomes very indistinct and appears 

 like mesenchyme. This is perhaps the origin of Jourdain's 

 statement that the mesoblast arises by delamination between 

 the ectoderm and endoderm, and not by archcnteric diver- 

 ticula. 



The gradual elongation and narrowing of the embryo 

 causes the obliteration of the cavities, and the whole embryo 

 becomes solid, as described by Hertwig. As it grows in 

 length it curls in the shell, and this ventral curvature 

 becomes more pronounced as development proceeds, so that 

 in S. bi punctata the tail meets the head (PL 19, fig. 6), and 

 in S. euflata the embryo is curled through fully a turn and 

 a half before hatching. In the latter species, in which the 

 shell fits the egg, the curvature begins much earlier than in 

 the large-shelled species. When the embryo begins to curl 

 it is easy to get optical transverse sections showing the 

 cephalic mesoderm at the sides of the stomodceum in the 

 head, and in the trunk the two semicircular mesodermal 

 masses separated by the thin endodermal septum, which 

 expands somewhat dorsally and ventrally. My observations 

 on the origin of the ganglionic rudiments and the remaining 

 changes befoi'e hatching are so nearly in accord with 

 Hortwig's that it is not necessary to give them in detail. In 

 the species that I studied hatching took place at a time 

 varying from sixteen to fifty hours after the eggs were 

 laid, differing according to the tompci'ature and the species. 



