56 BULLETIN: MUSEUM OF COMPARATIVE ZOÜLOuY. 
place. The four entodermal 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 invaginating 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 2? in a dorsal and anterior direction, This cell, together with the 
animal pole, has moved a very slight distance toward the maeromere 
end of the egg. The animal pole now lies (Fig. 78) directly above the 
plane separating the cells 4%. and d'?? from die and d., instead of 
above the posterior margin of the cells d. and d., 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 entodermic 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. - e., 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. 79). 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 ds 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 spiudles do not lie in parallel planes, so that no single 
view can give a complete representation of their positions. Nevertheless, 
Figure 83 shows that the arrangement is distinctly bilateral. The cell 
des 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 
