30 bulletin: museum of comparative zoology. 



deformed bv the pressure to such an extent that the surface area ex- 

 posed resembles the section of a biconvex lens. In the dorsal layer 

 tlie 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 than the corresponding 

 cells of the other quadrants (Figs. 31 and 33). The ventral blastomere, 

 d^-'^, 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 the outer, 

 is convex ; anteriorly the cell is partly covered by the ventral cells of 

 the other quadrants, while the ventral end of cell rf^-^ extends a slight 

 distance ventrad of the middle dorsal portion of d^'^. 



The cell rf^-^ is distinguished from all the others by a further peculi- 

 arity. 1 have shown above (page 2-5 and Fig. 7) that in the eight-cell 

 stage there is a slight concentration of yolk material in the ventral 

 region of the cell c?*-\ 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 the 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 

 (Fi<Ts. 20, 22, and 23). At a time when the fourth cleavage is entirely 

 completed, the granules have withdrawn still farther from the posterior 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 rf^-\ the original finely granular 

 cvtoplasm 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, rf^-^, is next in size, then the dorsal cell, 

 d^*, while the cell of the third layer is the smallest in quadrant D. 

 The cells d^-^ and d""^ are veiy greatly compressed dorso-ventrally and 

 elongated laterally, so as to form irregular flat plates, extending from the 

 posterior surfiice of the egg 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 



