No. I.] THE EMBRYOLOGY OF CREPIDULA. 77 



meres is to me quite uncertain. I think it safe to conclude 

 that the mesoderm arises by a separation of protoplasm from 

 one of the macromeres." I find that these elevations are the 

 small yolk cells 4a, 4b, and 4c. They appear somewhat later 

 than the cell 4d, just as is the case in Crepidula. 



That these cells really belong to the same quartette as 4d is 

 shown not only by their position and method of origin, but also 

 by the fact that the macromere D does not divide again until 

 a very late stage, and then in a series of divisions which affects 

 each of the other macromeres and leads to the separation of a 

 fifth quartette (5a-5d) from the macromeres. And that all the 

 cells of the fourth quartette, like those of every other quartette, 

 are really homodynamous, is strongly suggested by the fact that 

 they are all yolk cells of about the same size, that they are 

 chiefly entoblastic (4a, 4b, 4c entirely so, and 4d raore than 

 half), and that the points in which 4d differs from the other 

 members of this quartette are probably due to the posterior 

 elongation of the body and the origin of bilateral symmetry.^ 



Since these fourth-quartette entomeres are smaller than any 

 other cells of the inner layer except the enteroblasts, I shall 

 call them the smaller enteroblasts . Like the mesentoblast, 

 4d, they are formed by a laeotropic division, and immediately 

 after they are separated they begin to rotate in an anti-clockwise 

 direction, until they come to lie in the furrows between the 

 macromeres, and in this position they are carried around to 

 the ventral side with the growth of the ectoblastic cap. At the 

 same time that these cells rotate to the left all the derivatives 

 of 4d also rotate in the same direction, and thus come to lie at 

 the posterior end of the second furrow; and what is more 

 remarkable, the whole ectoblastic cap is rotated with these cells 

 through almost 45°. There is here furnished another evidence 



1 In a previous paper ('92) I called attention to the fact that the cell 4d is 

 homodynamous with the other cells, 4a, 4b, and 4c of the fourth quartette. Hey- 

 mons ('93), who reached the same conclusions in his work on Umbrella, curiously 

 misinterprets me on this point. He says (p. 270): " Nach Conklin sollen dagegen 

 die primaren Darmzellen (the macromeres A, B, C, and D) in Ursprung und Lage 

 den beiden Urmesodermzellen entsprechen, eine Ansicht, die ich fiir Umbrella 

 entschieden zuriickweisen muss." A reference to my paper will show that I there 

 advanced exactly the view which was afterwards advocated by Heymons, and is 

 still further elaborated in this paper. 



