THE DEVELOPMENT OF LAMELLIBRANCHS. 



321 



right side in Fig. 161, is to become the anterior end of the 

 body of the adult. The figure therefore shows the half 

 of the egg which is to become the left half of the body. 

 In most lamellibranchs, and especially in Unio and Ano- 

 donta, the micromere (6), is hardly distinguishable from 

 the macromere (), and the egg, at this stage, is like Fig. 

 162, instead of like Fig. 161. 



In the oyster, this first stage of active segmentation is 

 followed, as it is in the sea-urchin, by a period of rest, dur- 

 ing which the divisions between the spherules (a, b, and c), 

 become almost obliterated. In Unio and Anodonta, and 

 in most marine lamellibranchs, the resting stages are hardly 

 recognizable, and the egg passes almost immediately from 

 one stage of segmentation to another, but in the oyster 

 the resting stages are well marked. The oyster egg, in 

 the first resting stage, is shown in Fig. 162. The macro- 

 mere (), and the anterior 

 micromere (6), are so com- 

 pletely fused with each other 

 that the line of separation is 

 invisible, while that which 

 separates the posterior micro- 

 mere (c), from the rest of the 

 egg is still distinguishable. 



FIG. 163. The same egg, ten 

 minutes later. 



a, b, and 'c. as in Fig. 161. 

 d, d. The new microuieres. FIG. 163. 



During the next stage of segmentation, the two micro- 

 meres (Fig. 163, b and c), again become sharply defined, 

 and each of them divides into two, so that we now have one 

 macromere (), and four micromeres (6, c, d, d). In Unio, 



