46 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
Fig. 37). The single dorsal cell divided equatorially into a small dorsal 
and a large ventral part. There is appended a diagram of the quad- 
rant D after this cleavage (Diagram IL). 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 de. and die have 
passed inward, becoming the entoderm, so that we may omit them from 
the present discussion. Of the other cells, the ventral pair (d? and de.) 
R 
2 4 
" 
7 I 
$ 
AY. V 
Diacram II. Dracram III. 
Diagram of quadrant D in the sixth Quadrant D in the seventh genera- 
generation. Only the second exponent tion. Only the second exponent is 
designating the cells appears in the expressed, the first being in all 
diagram, the first being in all cases 6. cases T. 
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. 
divide equatorially, d*-? unequally, d** equally (see Plate 7, Figs. 57 
and 58). The two next layers divide meridionally (Fig. 57), the cell 
die 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 
