i'o 



tlu' ()l)eiiin>r. So tliat the embryo now consists of two liollow sacii: 

 the inner one, tlie digi stive trfiet. eoniniunieating with tile exterior 

 by a small pore — the primitive moutli. and surrounded at all 

 points except at the mouth by the outer sac Avhich fonns the 

 body wall (Plate II, Fig. 8). 



This stage in development, in which tlie riiil)i-y.. consists 

 of two hiycfs. the inner foniiiiig tlic di-cstivc tfact which is 

 enclosed witliiii an outer one wiiich forms the body wall, is of 

 very genci-al occufiviuH' in llic developmental history of widely 

 separated groups of animals, and is known as the (jastrula stage. 

 Some tinie after the free-swinnning stage has been reached, 

 a small, irregular plate of cells makes its appeai-anee on each 

 side of the body. These little plates are the shell glaiuls in which 

 the primitive shells have their beginnings. The shells are sepa- 

 rate from the first, and during the early development of the 

 oyster they are synnnetrreal and nearly circular in outline. 



After the shells make their appearance, the embryos cease 

 t.> crowd to the surface of the water, although they are still 

 capable of swimming about actively. The anterior part of the 

 body, which bears the cilia, begins to be sharply defined as a 

 circular pad— the voluni (Figs. 8-11), which serves as the organ 

 of locomotion during the remainder of the free-swimming life 

 of the embryo. 



The two shells, at first too small to allow the body to be 

 withdrawn between them, continue to grow, until they become 

 large enough to cover up or even extend beyond the surface of 

 the body. By this time muscular fibres have developed in the 

 body wall. These are arranged so that they can draw the 

 body and the velum inside the shell. The surface of the body, 

 which lines the shell in either side, becomes converted into the 

 mantle lobes and a mantle cavity is formed between them. 



The primitive mouth becomes closed during the early stages 

 of development, and later a new opening, the permanent mouth, 

 i.' formed on the opposite side of the body from the location of 

 the primitive mouth. After the formation of the permanent 

 mouth, the digestive cavity increases in size, and its walls be- 

 come covered with cilia. The wall of the stomach becomes con- 

 nected with the body wall a little way behind the mouth, and 

 another opening, the anus, is formed. The tube which connects 



