JENNINGS: DEVELOPMENT OF ASPLANCHNA HEERICKII. 55 



considerable shiftings which take place. As seen in Figure 65, the 

 animal pole lies at the anterior margin of the cell c?®-^. 



Before the eighth cleavage takes place, the blastopore has become 

 closed and its anterior margin has begun to pass into the two-layer 

 condition, as previously described. The larger cell, <f*-\ divides before 

 its mate, by a spindle at right angles to the previous spindle. The 

 cleavage is equal, forming the two large right and left cells (P-^ and 

 cP''-^, shown in Plate 10, Fig. 80. The plane separating these cells 

 coincides with that separating the quadrants A and B on the anterior 

 side of the egg, and also with that separating the two cells d^'^^ and 

 <p-^^ on the posterior side (Piute 8, Fig. 68). As this plane also 

 passes through the animal pole and the blastopore, it divides the egg 

 into symmetrical halves, and is the median dorso-ventral or sagittal 

 plane of the embryo. 



Later the small cell, d^-"^, develops a spindle in the same direction as 

 the spindle of the seventh cleavage, in the shorter axis of the cell, and 

 divides into two very unequal cells. The anterior or ventral cell, d^-^, 

 is a minute vesicle, whereas the dorsal or posterior blastomere, d^''^, is 

 scarcely smaller than the mother cell. The process of budding off this 

 small cell is shown in Plate 10, Figui'e 80. The vesicle lies between 

 the two large cells d'^-'^ and c?^-^, and, like the minute cells d^-^ and d''-'^, 

 can be traced but a short distance, soon becoming lost among the many 

 cells by which it is surrounded. 



In the ninth cleavage, spindles are formed in the cells d^'^ and d^'"^ in 

 the position foreshadowed by the asters in Figure 80, — that is, antero- 

 posterior, and at right angles to the preceding spindles, — and the cells 

 divide equall}^, forming the four cells d^^-^, c?^°*^, rf^''-^ and of^°-''. Figures 

 76 and 81 show" the entoderm at the close of these divisions — the 

 in;clei of the cells in question being still connected in pairs by inter- 

 zonal filaments. The blastopore is now present as a distinct notch ; its 

 anterior or dorsal lip has become two-layered, owing to the folding 

 inward of the cells of the anterior quadrants. The animal pole (jpol. 

 anm., Fig. 76) has moved a considerable distance on to the previously 

 anterior sui-face, lying still at the anterior margin of the entoderm cell 

 c?^'*. As the side view (Fig. 76) shows, a frontal plane carried through 

 the long axis of the Qgg at this stage would cut the nuclei of all the 

 entoderm cells, as i-ealized in the frontal section, Figure 81 (Plate 10). 



As soon as the cells, after the cleavage process is entirely finished, 

 have lost their strong tendency to maintain a form as nearly spherical as 

 possible, a sudden and considerable change of relative position takes 



