No. I.] THE EMBRYOLOGY OF CREPIDULA. 141 



periphery, Fig. 74. This proliferating area then invaginates, 

 forming at first a shallow depression, and later the edges of 

 this depression arch over until they nearly meet in the centre. 

 Figs. 75, "JJ, 92. A short time after this the edges begin to 

 extend in every direction, the invagination becomes shallower 

 and broader, and at the same time a thin cuticle, which is the 

 first trace of the shell, appears over the surface of the invagi- 

 nated cells. Figs. ']'&, 95, 104, 105. While the shell gland is 

 comparatively small, these cells are columnar, Figs. 92, 95, but 

 as it increases in size they become extremely flat and thin, so 

 that it is scarcely possible to see them even in sections, Figs. 

 103, 105. The cells at the margin of the shell gland, however, 

 are columnar, and it is from these that the growth of the shell 

 takes place. 



Owing to the shifting of the posterior end of the embryo 

 toward the ventral side, as shown in Figs. 80, 93, 95, the shell 

 which was at first on the postero-dorsal area comes to be located 

 entirely at the posterior end of the embryo, which now appears 

 truncated. Figs. 80, 93, 95. The margin of the shell gland 

 then extends forward on the left side much more rapidly than 

 it does on the right, and at the same time the whole posterior 

 part of the embryo is pushed over to the right. 



In both Neritina and Umbrella the shell gland forms on the 

 postero-dorsal surface of the embryo, but in neither case has 

 its cell origin been determined. In Unio it is derived from 

 the cell 2d, or the " first somatoblast," and, as we have seen, it 

 probably comes from the same cell in Crepidula. 



In Fulgur the invagination of the shell gland occurs at an 

 early period, when the ectoblast has extended but a short dis- 

 tance over the yolk. Its early appearance seems to have 

 misled McMurrich ('86), who regarded it as an invagination of 

 unknown significance, but of very general occurrence. Except 

 for its early appearance it is in all respects similar in origin 

 and development to the shell gland of other gasteropods. 



(e) The Foot. — Immediately after the formation of the shell 

 gland the foot appears as a single median protuberance on the 

 ventral surface, Figs. 'j(>, yy. At first the prominence is 

 about as long in the antero-posterior diameter as it is wide, but 



