No. I.] THE EMBRYOLOGY OF CREPIDULA. 125 



Comparisons. 



Blochmann ('81) has described the first division of the third 

 quartette in Neritina (according to his designations the cells are 

 a2, -bJ^ 2, \>2 and b^^2, etc., but following the modification which 

 I have suggested, they are, in the nomenclature of this paper, 

 3ai and 3a2, 3bi and 3b2, etc.). The cells lie closely pressed 

 into the furrows between the macromeres ; their first cleavage 

 is apparently radial, and the lower cell product (3a2, etc.) is 

 smaller than the upper (Blochmann's Figs. 51 and 53). In 

 passing, I merely call attention to the fact that this cleavage 

 closely resembles the first division of the third quartette in 

 Crepidula. 



Blochmann gives no further history of these cells in Neri- 

 tina, but his Fig. 56 shows what I believe is the second division 

 of the third quartette. If my interpretation of this figure, 

 which is given in Diagram 12, b, is correct, the upper cell in 

 each quadrant (3ai, 3bi, 3C1, 3di) divides bilaterally in each 

 case, giving off a smaller cell toward the mid line of the 

 embryo. This division is in most regards like the correspond- 

 ing one in Crepidula, but with this interesting point of difference : 

 in Neritina all the cells of this quartette, on the anterior as well 

 as on the posterior side, divide bilaterally. As in Crepidula, bilat- 

 erality appears most marked in the cells of the third quartette, 

 but it is characteristic of all the cells of the quartette, and not 

 merely of those of the two posterior quadrants. 



The further history of these cells cannot be followed in 

 Blochmann's figures. The following table presents a summary 

 of the cleavages of the third quartette in Neritina : 



3a 



< 



3a'(U|i|ier) 

 5 a^ (Lower) 



K3a''' (pinner) 

 3a '■=^- (Outer) 



The divisions of the third quartette in Umbrella are wonder- 

 fully like those in Crepidula. Heymons describes the direction 

 of the first division as follows (p. 253): "Die Spindeln sind 

 rechtwinkelig zur Dorsoventralachse des Eies gestellt," i.e., as 



