224 HELEN DEAN KING. 



removed from the chamber the eggs were in the 8-cell stage, but 

 development stopped at this point and all of the eggs were dead 

 inside of twenty -four hours. 



Experiment p. In this experiment, eggs in the 2- and in the 4- 

 cell stages of development, were subjected to a temperature of 

 3 3-3 5 C. for a period of two hours. At the end of this time the 

 eggs were segmenting very irregularly in the upper hemisphere 

 and no cleavage planes were visible in the yolk portion of the egg. 

 A section through one of these eggs (Fig. 3) shows the entire 

 upper hemisphere divided into a mass of small cells containing a 

 considerable amount of pigment which is, for the most part, col- 

 lected in the middle of the cell around the nucleus. The first 

 cleavage plane has cut only partially through the yolk portion of 

 the egg, as its progress was evidently stopped at the beginning 

 of the experiment. There are no nuclei in the yolk portion of 

 the egg, and the many vacuoles show the injurious effects of the 

 heat. The mass of small cells in the upper hemisphere forms 

 a sort of cap on the unsegmented yolk and make it appear 

 as if the segmentation of the egg was meroblastic. This same 

 sort of abnormal cleavage has also been obtained by Hertwig 



(I, 2). 



According to the experiments in this series, eggs in the early 

 cleavage stages can endure exposure to a temperature of 31- 

 33 C. for a longer period than can the unsegmented egg ; yet 

 they are permanently injured by even a short immersion in water 

 at a temperature of 35. The maximum temperature for these 

 eggs, therefore, is not greater than that for the unsegmented egg. 

 Hertwig (4) has found that the maximum temperature for the 

 eggs of Rana fusca in the 8-cell stage of development is 2628, 

 which is 34 higher than that for the unsegmented egg. 



IV. EXPERIMENTS ON EGGS IN LATE SEGMENTATION AND 

 EARLY GASTRULA STAGES. 



Experiment 10. On April 1 8, fifty eggs in the 32-64-cell 

 stage of development were kept at a temperature of 31-33 C. 

 for two hours. Subsequently all of the eggs developed into 

 normal embryos and at about the same rate as did the eggs of 

 the control set. 



