ZOOLOGY 



SECT. 



A 



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, ent.) 

 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 bias to - 

 ccele, where they ac- 

 mes ^-"V ^-*^^ N ^ 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 molluscan 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 



FIG. 571. Early embryo of Anodonta. eh, vitelline 

 membrane ; ent. archenteron ; in. micropyle ; men. meso- 

 derm ; rk, polar cells ; sd, shell-gland ; si; lateral cells ; 

 7r, cilia. (From Korschelt and Heider's Embryology.) 



R 



B. 



50. 



mes. 



ertf. 



Fio. 572. Two later stages in the development of Anodonta. ent. archenteron ; mes. meso- 

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

 Embryology.) 



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

 triangular form, its ventral angles produced into incurved hooks 



