336 



CHAP. 



saddle-shaped plate with thickened edge, on the dorsal surface of the 

 larva. On this plate a thin horny secretion, the first rudiment 

 of the shell, appears. As the process of eversion takes place the in- 

 vagination of the endoderm goes on rapidly, and soon a sac is formed 

 whose wall is composed of large columnar cells, and which opens to 

 the exterior by a constricted opening, the blastopore (Fig. 265). 



B 



stom 



FIG. 265. Sagittal sections of embryos of Dreissensia polymorpha, showing the process 

 of gastrulation and the formation of the shell gland. (After Meisenheimer. ) 



A, stage in which the endoderm and the fchell gland are both beginning to be invaginated. B, stage 

 in which the iuvagination of the shell gland has reached its maximum. C; stage in which the blasto- 

 pore is closed and the shell gland is beginning to be evaginated. U, stage in which the shell gland is 

 completely evaginated and the stomodaeum is beginning to be formed, a.ji, apical plate ; M, blastopore ; 

 end, endoderm ; h.p, cells which will eventually form the liver ; M, primary mesoderm cell ; p.tr, proto- 

 troch ; s.g, shell gland ; atom, stomodaeum. 



The blastopore becomes shifted forwards and finally closed in the 

 position where the mouth afterwards opens. This forward shift seems to 

 be largely due to the growth of the band of small cells, x x - x 5 , derived 

 from X, which separated originally X and M 011 the posterior surface 

 of the embryo. This band thus comes to occupy the region immedi- 

 ately behind the mouth ; and as the fuot is later developed in this 

 region, it probably owes its origin to these cells. When the 

 blastopore has been completely closed, the stomodaeum originates as 



