1 68 CONK UN. [Vol. XI 1 1. 



pore (vegetative pole) to one side of the larva, which then 

 becomes the ventral face. This movement is usually accom- 

 panied by the closure of the blastopore from behind forward. 

 In some trochophores, however, especially the larvae of the 

 Mollusca, in which there is a large accumulation of yolk at 

 the vegetative pole, this pole may remain fixed in position 

 while the animal pole is shifted forward. The end result is 

 the same in the one case as in the other. In still other cases 

 the movement takes place at both poles; in fact in all cases it 

 is probable that both poles shift position a little. 



The result of this shifting is that the t.^^ axis is bent, the 

 angle growing greater and greater throughout development. 

 This bending of the o.^^ axis caused by the shifting of the 

 poles may all be referred ultimately to the greater and more 

 rapid development of the posterior side of the egg, and this 

 greater growth may be located more specifically in the group of 

 cells derived from the first and second somotablasts (2d and 

 4d), and is associated with the formation of the trunk. The 

 continued elongation of the trunk in annelids is accompanied 

 by the development of metameric segmentation ; in mollusks 

 it leads to a ventral curvature of the trunk associated with 

 the formation of the shell on the postero-dorsal surface, and 

 in gasteropods this curvature of the longitudinal axis is still 

 further complicated by the laeotropic torsion of the trunk 

 region posterior to the foot. 



In Crepidula the conditions are precisely like those described 

 above for ova with a large amount of yolk. The chief axis of 

 the ovum becomes the principal axis of the gastrula ; this axis 

 is at first a straight line, but in later stages its upper end is 

 carried forward through an angle of about 90°. In fact, this 

 bending of the embryonic axis is so great and sharp that it is 

 doubtful whether one should not regard that axis as altogether 

 destroyed. The longitudinal or antero-posterior axis runs from 

 the apical cells in front to the anal cells behind, and is perpen- 

 dicular to the earlier position of the embryonic axis. 



The blastopore closes in the middle of the ventral side. 

 Figs. 65, 66, and its posterior lip does not appear to grow 

 more rapidly than the anterior or lateral lips. This might 



