322 HANDBOOK OF INVERTEBRATE ZOOLOGY. 



Anodonta, and many marine lamellibranchs, the spherule 

 (b) , at this stage, is not constricted ofi* from a. This period 

 of activity is followed in the oyster by a second resting 

 stage, and the micromeres then divide by repeated fusion 

 into a cap of small ectoderm cells (Fig. 164, ec), which 

 almost completely eovers the macromere (a). At the same 

 time the direction cell is carried towards the anterior end 

 of the egg. Fig. 164 shows the oyster egg 

 about six hours after fertilization. 



Fig. 164. — The same egg, seven hours and eight 

 minutes later. 



a. Macromere. b. Micromeres. ec. Ectoderm. 

 g. Point where orifice of invagination is to be formed. 



In about thirty hours after fertilization, the macromere 

 of the oyster egg also begins to divide into smaller cells, 

 and forms the digestive layer, or endoderm. In about 

 thirty-six hours (Fig. 165), it becomes flattened, dorsally 

 and ventrally ; the endoderm {en), becomes pushed in 

 on one of the flat sides to form a saucer-shaped digestive 

 cavity with a wide mouth, the orifice of invagination [g); 

 a segmentation cavity is visible between the endoderm, and 

 the ectoderm (ec), and a few short cilia appear on the 

 outer surface of the ectoderm. In from thirty-six to forty- 

 eight hours, the oyster embryo assumes the form shown 

 in Fig. 166. A tuft of cilia, the velum (v), is developed 

 at the anterior end of the body, and the direction cell may 

 frequently be seen among the bases of the cilia. The 

 embryo now begins to swim actively, and finds its way to 

 the surface of the water. An optical section (Fig. 167, b), 

 will show that this embryo is the flattened embryo shown 

 in Fig. 165, folded on itself, in such a way as to carry the 

 endoderm (en), into the centre, and thus form a thick- walled 



