30 BULLETIN: MUSEUM OF COMPARATIVE ZOÖLOGY. 
deformed by the pressure to such an extent that the surface area ex- 
posed resembles the section of a biconvex lens. In the dorsal layer 
the compression is less ; the cells are triangular in surface view, and the 
dorso-ventral extent is greatest. 
All the cells of quadrant D are much larger tham the corresponding 
cells of the other quadrants (Figs. 31 and 33). The ventral blastomore, 
de, is much the largest cell of the egg, and occupies the entire ventral 
end at this period. Its position as shown by the section (Fig. 32) is 
worthy of careful attention. Its dorsal or inner surface, like tho outer, 
is convex ; anteriorly the cell is partly covered by the ventral cells of 
the other quadrants, while the ventral end of cell dë? extends a slight 
distance ventrad of the middle dorsal portion of d51, 
The cell d°" is distinguished from all the others by a further peculi- 
arity. I have shown above (page 25 and Fig. 7) that in the eight-cell 
stage there is a slight concentration of yolk material in the ventral 
region of the cell dt, where the yolk granules are a little larger and 
more numerous. At the division of d*!, this cloud of granules, as a 
natural result of its position, remains in the cell d** (Plates 2 and 3, 
Figs. 16 and 19). At the same time it becomes more distinctly differ- 
entiated. The granules composing the cloud increase in size and range 
themselves about tho periphery of the egg, next to its free surface 
(Figs. 23, 24, Plate 3, and Fig. 28, Plate 4). A narrow strip of the 
posterior margin of the free surface of the cell is without the granules 
(Figs. 20, 22, and 23). At a time when the fourth cleavage is entirely 
completed, tho granules have withdrawn still farther from the posterior 
margin of the cell, and show a tendency to concentrate at the free sur- 
face of the cell over its anterior half (Plate 4, Fig. 32). In the other 
cells, and in the remaining portions of 4, the original finely granular 
cytoplasm is retained, so that I have not thought it necessary to repre- 
sent in the figures the yolk conditions in any region except where the 
cloud of granules is present. 
The cell of the second layer, d*?, is next in size, then the dorsal cell, 
dend, while the cell of the third layer is the smallest in quadrant DÐ. 
The cells d^? and d*? are very greatly compressed dorso-ventrally and 
elongated laterally, so as to form irregular flat plates, extending from the 
posterior surface of the ege two thirds of the distance to the anterior 
surface. (Compare Figure 31, left posterior surface, with Figure 32, sec- 
tion.) The dorsal cell d** is likewise compressed dorso-ventrally, so as 
to appear in a sagittal section (Fig. 32) as a low triangle. 
As a whole, the form and arrangement of cells are far from what is 
