56 bulletin: museum of comparative zoology. 



place. The four entoderraal cells, d^°-^-d^°-* become much flattened 

 dorso-ventrally, and the invagination of cells at the anterior lip of the 

 blastopore increases in extent. These cells press most strongly upon the 

 anterior surface of the ventral entodermal cells, forcing them toward 

 the posterior side. An early stage in the process is shown in Plate 9, 

 Figure 77. Two of the cells of the invaginatiug ectoderm have flattened 

 themselves against the entoderm cells in such a way as to form a direct 

 continuation of the longitudinal series of interior cells. This longitudinal 

 series is however tending to become curved by the displacement of the 

 cell d^-* in a dorsal and anterior direction. This cell, together with the 

 animal pole, has moved a very slight distance toward the macromere 

 end of the egg. The animal pole now lies (Fig. 78) directly above the 

 plane separating the cells c?^°-^ and d^°-^ from d^°-^ and c?^°-^, instead of 

 above the posterior margin of the cells d^°-^ and c?^°-*, as previously. A 

 frontal plane carried through the long axis of the egg would not now cut 

 the cell d^-* at all. Figure 82 shows a view from the animal pole, the 

 outer layer of ectoderm cells being supposed to be removed from the 

 dorsal half of the egg, while the entoderm cells remain in position. 



This process of rotation of the eutodermic contents, as one might call 

 the phenomenon, continues still farther. Figure 79 shows a stage in 

 which the process is much more advanced. The ectodermal plug at the 

 anterior lip of the blastopore has become very much thickened, and pro- 

 jects farther posteriad; the four large entoderm cells d^^-'^-d^"-*, of which 

 of course but one pair is shown in the side view, are now so displaced 

 that the plane separating the pair, which previously lay in the short axis 

 of the egg (Figs. 76 and 78), now lies in the long axis (Fig. 70). The 

 line connecting the centres of a given lateral pair is now at right angles 

 to the line previously connecting them. 



At this time the five large cells constituting the entoderm — the 

 minute cell d^-^ not being traceable farther — begin to undergo karyo- 

 kinetic changes preparatory to division. The spindles in various stages 

 are seen in three of the cells in the side view. Figure 79 ; the same 

 stage, slightly earlier, is shown from the animal pole in Figure 83 

 (Plate 10), in which the covering ectoderm is supposed to have been 

 removed from the dorsal side of the egg. As a comparison of Figures 79 

 and 83 shows, the spindles do not lie in parallel planes, so that no single 

 view can give a complete representation of their positions. Xevertheless, 

 Figure 83 shows that the arrangement is distinctly bilateral. The cell 

 d^* lies in the middle line, with its spindle in the sagittal plane of the 

 egg ; the spindles in the other cells all radiate outward from the region 



