No. I.] THE EMBRYOLOGY OF CREPIDULA. 71 



greatest care throughout the stages shown in the table above, 

 and in Figs. 22-41 in the plates ; and beyond this stage I 

 have traced the enteroblasts and the mesoblastic teloblasts step 

 by step, until the latter give rise to the mesoblastic bands 

 extending half way around the ^^^, Figs. 49, 51, 53, and the 

 former apparently become the distal portion of the intestine, 

 Figs. 61, 65, 6Z. 



Owing to difficulty in tracing these cells I was not able to 

 work out their history satisfactorily for a long time. Conse- 

 quently in both of my previous papers on this subject ('91 and 

 '92) I described the cell 4d as the primary mesoblast, feeling 

 assured that it was this because it gave rise to mesoblastic 

 bands. It was not until I had taken up the later history of the 

 entoblast and the formation of the alimentary canal, that I 

 found that the two proximal (originally posterior) cells in each 

 band were the first intestinal cells, and that therefore the cell 

 4d contained both mesoblast and entoblast. 



So far as I know, but two other cases at all similar to this 

 are said to occur among Mollusca. In Patella, Patten ('86) has 

 described two " entomesoblast " cells, which lie one on each 

 side of the four large cells at the vegetal pole. These large 

 cells are entoblastic, and probably correspond to the four 

 macromeres present in many other forms. The entomesoblasts 

 appear at the blastula stage, and after elongating into the cavity 

 of the blastula, each cuts off a large cell at its inner end, which 

 is the mesoblastic teloblast ; the outer part of each cell is 

 entoblast. 



The other case is given by Stauffacher ('93) for Cyclas 

 cornea. In this animal the formation of the mesoblast is simi- 

 lar to the process in Patella, except that the two mesentoblasts 

 are in contact along the mid line as they are in Crepidula, 

 whereas they are said to be separated by the four macromeres 

 in Patella. In both these cases, however, the resemblance to 

 Crepidula extends no farther than the formation of paired 

 mesentoblast cells, and even in this regard the resemblance is 

 more apparent than real. 



In the formation and subsequent divisions of the cell 4d, 

 Umbrella is strikingly like Crepidula. This resemblance is 



