226 HELEN DEAN KING. 



yolk cells that contain little, if any, pigment. In this egg, however, 

 the upper wall of the segmentation cavity is made up of a single 

 layer of heavily pigmented cells which are fully as large as any 

 other cells in the egg. Below the segmentation cavity, a por- 

 tion of the yolk is divided into a number of small cells, many of 

 which contain pigment massed around the nucleus. Some of 

 these cells are rounded and seem to lie free in the segmentation 

 cavity, an appearance also noted by Hertwig (4) in eggs oiRana 

 fitsca that were exposed to a temperature of 2935 C. after 

 having reached about the loo-cell-stage of development. 



Morgan has also noted the relatively large size of the cells in 

 the upper hemisphere of gastrulating eggs of Rana palustris that 

 had been subjected to cold. He suggests that this increase in 

 the size of the cells " may be due in part to the absorption of 

 water by the individual cells," and he adds that, " even if this is 

 the case the cells are fewer in number than in a normal egg 

 beginning to gastrulate." In the figure shown by Morgan, 

 the cells of the lower hemisphere are all considerably larger 

 than those of the upper hemisphere ; the egg, therefore, must 

 have been much more normal than the one from which Fig. 4 

 was drawn. 



It is evident, in the case of the egg shown in Fig. 4, that the 

 increased temperature did not injure the yolk region or retard its 

 development as is usually the case in these experiments ; on the 

 contrary, it is the segmentation of the upper hemisphere that has 

 been delayed, while the segmentation of the lower portion of the 

 egg has continued. No egg in this set of experiments developed 

 much beyond the stage represented by Fig. 4, and each of the ten 

 eggs that were sectioned showed abnormalities of the same gen- 

 eral character. 



Experiment ij. On April 26, about seventy-five eggs in the 

 late blastula stage were subjected to a temperature of 3335 C. 

 A part of the eggs were removed at the end of one and one-half 

 hours and they all developed into normal embryos. 



A second portion of the eggs was exposed to this temperature 

 for two and one-half hours. All of these eggs developed into 

 normal embryos, although somewhat more slowly than did those 

 of the control set. 



