354 



MOLLUSC A. 



tion to fresh-water conditions. Moreover, a temporary parasitism of 

 the larva has complicated the later stages. 



The egg cell is surrounded by a vitelline membrane, and attached to 

 the wall of the ovary by a minute stalk, the insertion of which is marked 

 on the liberated ovum by an aperture or micropyle, through which the 

 spermatozoon enters. 



Segmentation is total but unequal. A number of small clear yolkless 

 cells are rapidly divided off from a large yolk-containing portion, which 



id 



Fig. 154. — Development of Anodonta. — After Gcette. 



x. Section of blastosphere. s.d., Shell gland : c.d., ciliated disc ; e., 

 beginning of ectodermic invagination. Note mesoderm cells in 

 the cavity. 



2. Later stage, in., Mesoderm. 



3. Embryonic shell has appeared. 



4. Glochidium larva ; note byssus threads, and teeth on shell 



valves. 



is slower in dividing. Eventually » hollow ball o- cells or blasto- 

 sphere results (Fig. 154). 



On the posterior dorsal region a number of large opaque cells form 

 an internally convex plate, — the beginning of the future shell-sac. A 

 pair of large cells are intruded into the central cavity, and begin the 

 mesoderm. 



On the under surface posteriorly there is a slight protrusion of ciliated 

 cells forming a ciliated disc. In front of this, at an unusually late stage, 

 an invagination establishes the archenteron, and the embryo becomes a 

 gastrula (see Fig. 154). 



The shell-sac forms an embryonic shell, and many of the mesodeim 



