54 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
fering fundamentally in kind from those already shown in the earlier 
Stages. 
We must now turn to a consideration of 
Tue ENTODERNM. 
The cells which I have called the entoderm — following in this Zelinka 
(91) and Tessin (86) 
the quadrant D, which passes within the egg in the manner already de- 
are those derived from the single large coll of 
scribed. I have already given an account of the first cleavages of this 
cell after it has become partially covered by the other blastomeres. As 
will be recalled, at the fifth and sixth cleavages the spindles occupied 
twice in succession the same position, one end lying in the anterior 
median line, between the ventral cells of quadrants 4 and B (Plate 5, 
Fig. 35, and Plate 6, Fig. 48). Here was given off at each of these 
cleavages a minute vesicle, the entire process being comparable in ex- 
ternal features to the successive formation of two polar cells at a given 
spot on the surface of the egg. The two vesicles thus formed maintain 
their position for some time (Plate 7, Fig. 55), but as tho surrounding 
cells become invaginated, I have found it impossible to follow their 
later fate. 
We will follow the cleavage of the large cell 4% (Plate 6, Fig. 50), 
which forms the greater bulk of the entoderm. 
After the sixth cleavage (Fig. 49), the asters in d™ at once separate 
nearly in the dorso-ventral axis of tho egg, as shown in Figure 50. The 
line joining them is at first a little oblique, the ventral aster being : 
little to the right. This obliquity soon corrects itself, and the asters 
come to lie in the sagittal plane. As the spindle is formed, its dorsal 
end moves to the posterior side, so that the spindle is no longer in the 
dorso-ventral axis of the egg. This stage is shown in Figure 64 (Plate 
8); as may be observed in this figure, the spindle is neither in the 
longer axis of the cell nor at right angles to it, but oblique. The cloud 
of granules, which soon after the last division oceupied a region on the 
anterior side of the cell, underneath the two vesicles d? and , now 
surrounds the aster at the dorso-posterior end of the spindle. 
The cleavage is unequal, separating (Fig. 65) a smaller dorso-posterior 
cell, d*?, from a larger anterior one, d®t, The cloud of granules 
remains in the smaller, dorsal cell, forming a band about its periphery, 
so as to leave a free space surrounding the nucleus. The position of 
the animal pole of the egg with reference to this cell should be carefully 
nofed, as the relation remains constant, at least for a. time, during the 
