No. I.] THE EMBRYOLOGY OF CREPIDULA. 155 



the posterior part of the cells C and D, and lie at nearly the 

 same level with the parent cells. This is especially true of 

 the cell 5C which is formed some time before the correspond- 

 ing cell 5D. The right side of the gastrula after the forma- 

 tion of 5C is accordingly much longer than the left, i.e., the 

 posterior end of the second furrow is carried far to the left 

 by the growth and horizontal division of the cell C (Figs. 

 58, 59, in which right and left are reversed because seen 

 from the ventral side). With the formation of 5D the left 

 side of the gastrula becomes a little longer and the posterior 

 end of the second furrow is carried back to the right, Fig. 60 ; 

 but either because the cell 5D is smaller than 5C or because 

 it is given off somewhat more toward the ventral side, the left 

 side of the gastrula remains permanently shorter than the right. 

 This is the beginning of the final asymmetry of the gasteropod. 

 It is also the beginning of the antero-posterior elongatio7t of the 

 body. Before the formation of the fifth quartette the trans- 

 verse diameter of the embryo is as great as the median; after 

 the cells 5C and 5D are formed the median diameter is dis- 

 tinctly longer. Figs. 59, 60, 64, et seq. This antero-posterior 

 elongation is due, then, in the first instance to the direction of 

 the cleavage in the posterior macromeres C and D, while the 

 first trace of the final torsion is due to a difference in the time 

 and direction of the cleavage in these two cells. 



At the same time that these changes in shape are taking 

 place other changes are going on which lead to the formation 

 of the archenteric cavity, as shown in Figs. 87, 88, 90, which 

 represent actual sections through the egg at this period. Fig. 

 87 is a transverse section showing two macromeres in the mid- 

 dle and two cells of the fourth quartette, probably 4A and 4C, 

 at the sides ; these cells are in process of passing around to 

 the ventral side of the macromeres. Fig. 88 is an oblique 

 vertical section, and shows two cells of the fifth quartette 

 lying on the ventral side of the macromeres and traces of the 

 fourth quartette cells beneath these. Finally, in Fig. 90, which 

 is a transverse section, there is shown in order from above 

 downwards two macromeres, two cells of the fifth quartette, 

 and two cells of the fourth quartette. Between these cells is 



