DEVELOPMENT AND LIFE HISTORY. 



383 



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 



FIG. 190. Development of Anodonta. After Goette. 



1. Section of blastosphere. s.rf., Shell gland ; c.d., ciliated disc ;e., 



beginning of ectodermic invagination. Note mesoderm cells in 

 the cavity. 



2. Later stage. ?., Mesoderm. 



3. Embryonic shell has appeared. 



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



valves. 



is slower in dividing. Eventuallv a hollow ball of cells or blasto- 



o * 



sphere results (Fig. 190). 



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. 190). 



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



