90 



is much like that of the latter. Since we have been led 

 along so far it will perhaps be profitable to inquire into 

 the next phase of the oyster as throwing light upon the 

 origination of the permanent mouth, stomach and intes- 

 tines. Fig. 11, which I have copied from Salesnky, rep- 

 resents the larva of the Euro- 

 pean oyster at a stage consid- 

 erably farther advanced than 

 my Fior. 10 of the clam. I have 

 inverted the original and 

 ^changed the lettering some- 

 ' what, in order to make the 

 ^^^p, figure correspond in position 

 and to make the names of the 

 parts agree with the nomencla- 

 Figure 11. ture which I use. 



The depression at Hy. in Fig. 10, has disappeared in 

 Fig. 11 and the shell /S" covers the spot. It will be noticed 

 too, that the hypoblast Hy. has arranged itself anew and 

 has been broken up into smaller cells and is completely 

 covered by the e.piblast Ep. The Hypoblast is also be- 

 ginning to show signs of becoming hollow in the center, 

 and a true archenteron has probably been defined, but as 

 yet there is no communication wdth the outside. It will 

 also be noticed that the hypoblast, as a Avhole, has been 

 pushed farther up relatively than in Fig. 10, and that it 

 is in close contact with the epiblast at M and A . Sin- 

 Singularly enough, the first evidence of a mouth appears 

 as a depression in the epiblast at a point nearly opposite 

 >S' / the depression M slowly deepens and finally breaks 

 through, and is brought into communication with the 

 archeuteric cavity when not long after the vent breaks 

 through at J , when the larval stomach and intestine are 

 com]3lete. By this time the velum Tis developed, ena- 

 bling the embryo to swim about by means of the rapid 



