46 



BULLETIN: MUSEUM OF COMPxVKATIVE ZOOLOGY. 



Fig. 37). The single dorsal cell divided eqiiatorially into a small dorsal 

 and a large ventral part. There is appended a diagram of the quad- 

 rant D after this cleavage (Diagram II.). If the septa in the three 

 ventral cells were moved to the middle of the cells, the diagram would 

 represent the condition in any one of the other three quadrants. 



At the next division (sixth) the two ventral cells d^-^ and d^-'^ have 

 passed inward, hecoming the entoderm, so that we may omit them from 

 the present discussion. Of the other cells, the ventral pair (d^-^ and d^-*) 



Diagram IL Diagram III. 



Diagram of quadrant D in the sixth 

 generation. Only the second exponent 

 designating the cells appears in the 

 diagram, the first being in al! cases 6. 



The arrows connect cells of common origin, and show the direction of the spindles 

 at the preceding division. R signifies right; L, left; D, dorsal; V, ventral. 



Quadrant D in the seventh genera- 

 tion. Only the second exponent is 

 expressed, the first being in all 

 cases 7. 



divide equatorially, d^'^ unequally, d^'^ equally (see Plate 7, Figs. 57 

 and 58). The two next layers divide meridionally (Fig. 57), the cell 

 d^-^ unequally, the others equally. The dorsal cell divides equatorially 

 and unequally. Diagram III. shows the ectodermal part of this 

 quadrant at the end of the sixth cleavage. The actual condition in the 

 egg at this period is shown in Figure 58, and at a slightly earlier stage 

 in Figure 57. 



Comparison of Diagram III. with the type diagram for the other 



