No. I.] THE EMBRYOLOGY OF CREPIDULA. 129 



(a) Blastopore, Stomodaemn, and Mouth. — The ectoblast 

 extends over the yolk from all sides at about the same rate, 

 in consequence of which the blastopore closes near the middle 

 of the ventral side. However, the growth of the ectoblast 

 does not take place at the same rate in all directions, as is 

 clearly shown by the fact that the apical cells do not lie 

 opposite the blastopore at the time when it closes, but at the 

 anterior end of the embryo, Fig. 65. The angular distance 

 between the apical cells and the blastopore in this figure is less 

 than 90° on the anterior side, while it is more than 270° between 

 the same points in the opposite direction. This greater growth 

 of the ectoblast on the posterior side of the egg must take 

 place entirely in the cells of the second and third quartettes, 

 since at this stage the growth of the first quartette is greater 

 anterior to the apical cells than posterior to them. This un- 

 equal growth does not cause the blastopore to close more- 

 rapidly at its posterior side, but it does change the position 

 of the apical pole, though the ventral pole remains fixed until 

 after the closure of the blastopore. 



At first the blastopore is not circular in outline; in fact, from 

 the time when the germ layers are fully segregated, Figs. 42, 

 43, the cells of the third quartette in each quadrant lie slightly 

 nearer the ventral pole than those of the second. This 

 advance is somewhat increased in Fig. 47, so that the outline 

 of the edge of the ectoblast is notched at four points, corre- 

 sponding to the second quartette cells, and protrudes at the 

 four intermediate points, which correspond to the third-quar- 

 tette cells. In a later stage. Fig. 52, when the advancing 

 edge of ectoblast can be seen from the ventral side of the Q.%g, 

 its notched character is still more apparent, the four notches 

 forming the angles of a regular quadrangle. The angles of 

 this quadrangle lie over the first and second cleavage furrows ; 

 at the posterior angle there is a broad recess in the lip of the 

 blastopore, and within this recess are the four enteroblasts 

 still uncovered by the ectoblast (see also Fig. 48).^ 



1 The quadrangular form of blastopore has been described by Wilson in Nereis ; 

 and in that animal, as in Crepidula, the angles lie within cell groups derived 

 from the second quartette, while the sides are formed by cells of the third quar- 

 tette. 



