EFFECTS OF HEAT ON TOAD'S EGG. 225 



Experiment n. Another set of fifty eggs from the same bunch 

 as the eggs used in experiment 10, was subjected to a tempera- 

 ture of 3 1-33 C. for three hours. The late segmentation and 

 early gastrulation stages of all of these eggs seemed to be perfectly 

 normal. Two days after the experiment was made, 38 of the 

 eggs were dead, the blastopore not having closed in any case. 

 Of the remaining eggs four only were normal, the rest had a 

 large yolk plug at the posterior end of the body. 



Experiment 12. - - Twenty-five eggs from the same lot as those 

 used in the two preceding experiments remained in water at a 

 temperature of 31-33 C. for three and one-half hours. Fifteen 

 of the eggs died in the blastula stage. The blastopore appeared 

 in the other ten eggs, but in many cases it was in an unusual 

 position at the equator of the egg. When the dorsal lip of the 

 blastopore was forming in these eggs, the circular blastopore 

 was already beginning to close in the control set of eggs, there- 

 fore, in this instance, the heat retarded instead of increased the 

 rate of development of the eggs. In none of the eggs of this 

 set did the blastopore ever become circular, and all of the eggs 

 were dead two days after the experiment was made. 



Fig. 4 shows a section of one of these eggs preserved when 

 the blastopore appeared in surface view as a short, straight line 

 at the equatorial zone. The dorsal lip of the blastopore rarely, 

 if ever, comes in as high up as the equator in eggs that are de- 

 veloping normally ; but it sometimes occupies an unusual posi- 

 tion in eggs that have been subjected to abnormal conditions. 

 Morgan (5) has found the blastoporic rim above the equato- 

 rial zone in eggs of Rana pahtstris that have been subjected to 

 intense cold. In Fig. 4 the archenteron appears as a shallow 

 depression with its dorsal wall formed of heavily pigmented cells 

 as is normally the case. The inner end of the archenteron, in- 

 stead of turning up towards the black pole as it would do in a 

 normal egg, here projects downward towards the yolk pole. 

 The most interesting fact shown by the section is that the normal 

 position of the large and of the small cells of the egg is com- 

 pletely reversed. In normally gastrulating eggs, the roof of the 

 segmentation cavity is formed of two to three layers of small, pig- 

 mented cells, while the ventral wall is composed entirely of large 



