Hi)2 



ZOOLDOY 



fd?--t, 



Development. — Segmentation of the oosperm is complete, but 

 unequal. A gastrula is formed by the invagination of the mega- 

 meres into the micro- 

 meres, but the archen- 

 teron (Fig. 571, eni.) 

 thus formed is quite 

 small and insignifi- 

 cant, and has no 

 physiological import- 

 ance until a late period 

 of larval life. Certain 

 of the cells of the 

 gastrula are budded 

 off into the blasto- 

 coele, where they ac- 

 cumulate and form the 

 mesoderm (mes.). At 

 about the same time a 

 deep invagination (sd.) 

 is formed, -which might 

 easily be mistaken for 

 the archenteron, but is 

 really a very characteristic moUuscan organ, the shell-gland : it 

 marks the dorsal surface of the embryo. The posterior end is 

 distinguished by a tuft of long cilia. 



The shell-gland becomes converted into a plate of long, cylin- 

 drical cells (Fig. 572, sd.), from which an unpaired shell (s.) is 



Fro. 571.— Early embryo of Anodonta. eh, vitelline 

 membrane ; ent. archenteron ; m. micropyle ; mc^. meso- 

 derm ; rk, polar eells ; sd, shell-Rland ; «/.-, latertxl cells ; 

 w, cilia. (From Korschelt and Heider's Eiuhryoiofjy.) 



E. 



B. 



FiC: 572. — Two later stages in the development of Anodonta. ent. archentfron ; mes. meso- 

 derm.; s. shell; sd, shell-gland; so. sense-organs; w, cilia. (From Korschelt and Heider's 

 Biiibryologi/.) 



secreted.. This is replaced before long by a bivaived shell of 

 triangular form, its ventral angles produced into incurved liooks 



