JENNINGS : DEVELOPMENT OF ASPLANCHNA HEEKICKII. 51 



place have commonly been already somewhat elongated in the direction 

 of the spindle. Thei'efore, at the occurrence of division, the short cells, 

 cleaving equatorially, have changed form greatly, becoming more elon- 

 gated dorso-ventrally, while the cells which cleave meridionally have 

 already been of sufficient length to permit of the extension of the spin- 

 dle without much change of form. As a result, there has been a great 

 extension of the ectoderm dorso-ventrally. This produces first a com- 

 plete closure of the blastopore. (Compare Figure 65, Plate 8, with 

 Figure 76, Plate 9.) A second result, due to the larger size of the 

 cells of the quadrant D, and perhaps partly also to the fact that they 

 cleave first, is the further displacement of the animal pole of the egg 

 from the micromere end toward the anterior side (Plate 8, Fig. 65, 

 Plate 9, Fig. 76). 



The closure of the blastopore is not sufficient to provide for the dorso- 

 ventral extension brought about during the cleavage, so that, as a third 

 consequence, the cells at the ventral pole of the ^^^, where the blasto- 

 pore was previously situated, are pushed over or under one another, 

 the ectoderm tending to become two-layered in this I'egion. A compari- 

 son of Figure 6.3 (Plate 8) with Figure 73 (Plate 9) shows the result- 

 ing conditions. In the latter figure the entoderm {d}-^ in Fig. 6.3) is 

 entirely enclosed, and the ventral cells cl'-^-c''^, a'''^-c''''-^, a'-^-c'-^, and 

 ^7.4_g7.4 hj^ve become crushed together, and several of them, as «^*^ and 

 b''-^, are almost hidden by suri'ounding cells. The beginning of the two- 

 layer condition is shown in frontal section in Figure 80 (Plate 10), 

 from the same egg as that shown in Figure 73 (Plate 9). 



Meanwhile other cleavages are taking place in quadrant D, leading to 

 still further modifications of the structui'e of the egg. It is therefore 

 necessary to return to a consideration of this quadrant. 



Quadrant D. — By reference to the diagram of the cells of this quad- 

 rant at the end of the seventh cleavage (Diagram IV., page 48), it will 

 be seen that there are now present, exclusive of the entoderm, twenty- 

 three cells, arranged somewhat irregularly. Approximately the same 

 stage, as it actually appears, is shown in Figure 68 (Plate 8). 



lu this egg spindles have appeared in the large cells d^'^^ andc?*'^^, and 

 later they each become divided into two equal cells. It is worthy of 

 notice that the two spindles are not parallel, but make a slight angle 

 with each other. The two cells lie on opposite sides of the median 

 dorso-ventral plane, so that the angle between the spindles indicates the 

 beginning of a tendency to bilateral cleavage. The inclined position of 

 the other spindles (in c''-'' and c''-") is indicative of the same fact. The 



