44. 



otocj'-sts, figure 56, ot. The ne:ct stage shows the otocj^sts ap - 

 parently completely closed off, lying yer:;^ close to the clefts 

 that separate the mantle from t2ie body wall. 



The formation of the mantle and shell is accompanied 

 by a slight lateral com^jression and a corresponding dorso-ventral 

 widening of the embryo. The test cells become yery much thinner 

 and the vacuoles, nearly or quite disappear, but tlie cilia re- 

 main as described and tiie moA'-ements of swinmiiig are not altered. 

 Several new organs ma^e their appearance. 



Cleared "iiS^ole motmts show some of the internal organs, 

 but throughout it is necessary to resort to reconstructions from 

 the study of serial sections, to get the relationship of the dif- 

 ferent parts. 



The body of the embrj'-o now lies between the shell valves 

 and these are enclosed hx the test. The foot, figures 33, 55 , 

 and 61, f, is formed by a rapid increase of cells between the mid- 

 gut and the stomodaeum. The anterior adductor muscle, figures 

 33, 55, and 56, aa, is formed at a corresponding time (about 85 

 hours) and consists of a few muscle fibers which stretch from^ 

 one shell valve to the other. It is left, by the gro7rth of the 

 embryo, separated from the rest of the bods'-, except at its ends, 

 which penetrate the majitle, and are attached to the shell. The 



