EFFECTS OF HEAT ON TOADS EGG. 223 



however, the eggs of the control sets appeared to be fully as far 

 advanced in development as were the eggs that had been subjected 

 to a higher temperature. Increase in the rate of development is, 

 therefore, but the immediate effect of heat, and after the eggs are 

 brought into a lower temperature they develop at the same, or a 

 lower rate, than the eggs of the control set. 



III. EXPERIMENTS ON EGGS IN EARLY CLEAVAGE STAGES. 



Experiment 6. On April 17, a lot of about fifty eggs in the 

 2-cell stage of development was exposed to a temperature of 

 31-33 C. At the end of one and one-half hours, part of the 

 eggs were removed. They were then in the 8 i6-cell stage. 

 The later development of these eggs was perfectly normal in 

 every respect. 



The rest of the eggs of this lot remained in the heated chamber 

 for two hours. All of these eggs developed normally during the 

 early cleavage and gastrulation stages ; but later a few embryos 

 were found with shortened medullary folds and a large yolk plug 

 at the posterior end of the body. This form of abnormality is 

 very common among embryos that have been injured by ex- 

 posure to heat. 



Experiment 7. As the first cleavage plane was appearing, a lot 

 of about fifty eggs was subjected to a temperature of 35-36 C. 

 for three quarters of an hour. All of the eggs were segmenting 

 in a very abnormal manner when they were transferred into water 

 at the room temperature, and none of them ever gastrulated. 

 Fig. 2 shows a median section through one of these eggs. With 

 the exception of the layer of small cells bordering the outer sur- 

 face of the upper hemisphere, the entire substance of the egg is 

 seen to be unsegmented and to have a number of different sized 

 vacuoles scattered through it. A large, irregularly shaped cavity 

 fills the greater part of the upper hemisphere of the egg. This 

 cavity is much larger than the segmentation cavity in a normally 

 segmenting egg, and it appears to be formed of the true seg- 

 mentation cavity and several large vacuoles which have come to 

 open into it. 



Experiment 8. On April 22, a lot of eggs in the 2-cell stage 

 was exposed to a temperature of 35-36 C. for one hour. When 



