336 



INVEETEBEATA 



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). 



Fig. 265. — Sagittal sections of embryos of Dreissensia polymorpha, showing th,e process 

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



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

 in wliich the invagination of tiie shell gland has reached its maximnm. C, stage in wliich the blasto- 

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

 completely evaginated and the stomodaenm is beginning to be formed. a.jj, apical plate ; hi, blastopore ; 

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

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



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, Xj - x^, derived 

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

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

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

 region, it probably owes its origin to these cells, \yhen the 

 blastopore has been completely closed, the stomodaeum originates as 



