89 



wliicli the stomach and intestine is to be developed. In 

 Fig. 9 the micromeres are slowly creeping down over the 

 megamere which is finally enclosed by the former grow- 

 ing in a single layer from above downwards. This mode 

 of developing a two layered stage of the embryo is known 

 as epiboly, meaning that the epiblast grows over and 

 upon the hypoblast which is here represented by the 

 megamere. The megamere is finally pushed inwards and 

 covered by the micromeres or cells of the epiblast. Once 

 fairly inside and included by the epiblast as shown in 

 Fig. 10, the megamere begins to divide as 

 shown by the darkly shaded cells in the 

 lower part of the figure. A depression ap- 

 pears in the middle of this cluster of cells 

 which must, on anatomical grounds, be con- Figure 10. 

 sidered the orifice of invagination of the hypoblast. This 

 orifice or blastopore may be considered the mouth of the 

 so-called gastrula or stomach- stage of development, but 

 it is never used as an inlet for food, which is not yet 

 usable by the larva. The depression however persists, 

 and there is every reason to believe that, like in the oys- 

 ter and fresh- water mussel, it is the point where the shell- 

 gland of the embryo develops and that it therefore marks 

 the position of the back of the animal. Fig. 10 also illus- 

 trates plainly the position of the outside skin layer or 

 epiblast Ep.^ from which the mantle, gills, and siphons 

 of the adult are derived, and its relation to the darkly 

 shaded hypoblast cells Hy. inside, which finally become 

 fully covered by the epiblast so as to appear like a clus- 

 ter of cells in the inside of the hollow sphere formed by 

 the latter. The central cluster may now be looked upon 

 as the archenteron or primitive foundation from which a 



true stomach is derived. 



The way in which the gastrula stage of the clam is 

 formed is so far unquestionably the same as that of the 

 oyster ; what lies beyond we cannot say, but it is in the 

 highest degree probable that the history of its later stages 



