52 bulletin: museum of comparative zoology. 



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 d^-^ (Fig. 68). Tlie cells resulting from the 

 division of rf«-" and (f*-^^ ^^.g ^9.21^ ^9.22^ ^9.23^ ^^^ ^.u . ^^^^^ ^re shown 



in Figure 74 (Plate 9). 



Next the four cells immediately dorsad of these, c?*-", d^-'^^, c?^-", 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 rf^-^, rf^-'®, d^-^ and d^-^^, shown in the dorsal 

 part of Figure 68 (Plate 8), cleave equatorially also. 



As a result of these many equatorial cleavages the quadrant D 

 becomes gi'eatly increased in dorso-ventral extent. The animal pole is 

 forced fai'ther 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 cP'^ and rf^-^° 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 YI. shows the condition at 

 this time. 



The dorso-ventral extension of the ectoderm, and consequent crowd- 

 ing together of the cells in the region of the blastopore, are still further 

 increased by the eighth cleavage of the large cells a^-^^-c^-^°, 08.26 _g8.26^ 

 g8.27_p8.27_ j^^(j ^8.28_p8.28^ belougiug to thc otlicr quadrauts, 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 happened 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 

 of a single layer of cells from the quadrant D, resting against the ento- 

 derm cells. Between these ventral cells of quadrant D 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 



