52 BULLETIN: MUSEUM OF COMPARATIVE ZOÜLOGY. 
tendency toward bilaterality is apparently due to the crowding of the 
cells from all directions toward the blastoporic region, which lies at 
the ventral border of dè? (Fig. 68). The cells resulting from the 
division of de and de! are d”, d. 22, d. 28, and dt; they are shown 
in Figure 74 (Plate 9). 
Next the four cells immediately dorsad of these, de. h, d, de, and 
d*-*, develop spindles in their short axes (Plate 8, Fig. 68), which as a 
result of extension become the long axes (Fig. 74, Plate 9), — the 
cleavage being in each case equatorial and equal. 
Now the four large cells de., d, d e and dee, shown in the dorsal 
part of Figure 68 (Plate 8), cleave equatorially also. 
As a result of these many equatorial cleavages tne quadrant D 
becomes greatly increased in dorso-ventral extent. The animal pole is 
forced farther upon the anterior side of the egg, toward the blastopore, 
so that the cells of the quadrant D come to occupy in the region of a 
sagittal section much more than half the circumference of the egg 
(Plate 9, Fig. 76). 
The two ventral cells dee and dee meanwhile divide meridionally, 
completing the separation of quadrant D into two portions lying on each 
side of the median line (Fig. 74). However, the egg is not yet com- 
pletely separated by cleavage planes into right and left halves, for the 
entodermic cell d? occupies the median plane at even a later stage than 
this (Plate 10, Fig. 83). 
This is the latest stage to which I have traced the cleavage in the 
ectodermal part of quadrant D. Diagram VI. shows the condition at 
this time. 
The dorso-ventral extension of the ectoderm, and consequent erowd- 
ing together of the cells in the region of the blastopore, are still furthor 
increased by the eighth cleavage of the large cells 8-25 _ 68.25, (48,28... 58,28, 
d. 27 eè and 48.28 ce, belonging to the other quadrants, which like- 
wise divide equatorially and equally (Plate 9, Fig. 75). 
The blastoporic region has now become distinctly two-layered, as 
shown in Figure 77. The cells of quadrants A, B, and C are turned 
in and pushed dorsad, in the same manner as bappened in early stages 
to the large ventral cell of quadrant D. The anterior lip of the blasto- 
pore thus becomes two layers thick, while the posterior lip is formed 
wg a € 
of a single layer of cells from the quadrant D, resting against tho ento- 
derm cells. Between these ventral cells of quadrant JD and the infold- 
ing cells of the other quadrants, a slight notch appears, marking the 
position of the blastopore. (At an earlier stage the blastopore was 
