134 HELEN DEAN KING. 



which lots of eggs were fertilized were 3 5° C. and 5° C. It is 

 highly improbable that in a state of nature the eggs of Bufo are 

 ever fertilized at these temperatures which are very injurious to 

 the unsegmented &%g, although they do not seriously interfere 

 with the development of segmented eggs unless allowed to act 

 for a comparatively long period of time (King, 3). Not more 

 than one half of the eggs subjected to extreme temperatures 

 were fertilized. Segmentation of these eggs did not begin until 

 some three hours after the experiments were started, and in a 

 majority of cases the cleavage planes came in very irregularly. 

 A considerable number of eggs that began segmentation died 

 within a {qsn hours, so that but a very small number of individuals 

 continued their development. After the eggs in these lots had 

 reached the gastrulation stage they had apparently outlived the 

 injurious effects of extreme temperature, as only three of the 

 tadpoles that developed from the eggs that were fertilized in 

 water with a temperature of 35° C. and but two from those that 

 were fertilized at 5° C. died before it was possible to ascertain 

 their sex. The great majority of eggs that were fertilized at 

 medium temperatures, 30° C, 20° C. and 10° C, segmented 

 normally and continued their development. The mortality in 

 these lots was very low, not more than 6—8 per cent, of the 

 individuals dying before it was possible to ascertain their sex. 

 The various lots of tadpoles received similar food and were all 

 kept as nearly as possible under similar external conditions 

 during their entire growth period. 



In this series of experiments, and also in several earlier ones, 

 many of the tadpoles died after beginning their metamorphosis. 

 In a considerable number of these individuals it was necessary 

 to section the gonads in order that sex might be ascertained. 

 The toads that survived metamorphosis were placed in large 

 glass jars containing moist sod, and they were fed on small 

 insects for several days. Young toads grow very rapidly, and 

 by keeping them in the manner indicated for a short time it is 

 possible to ascertain the sex of practically every individual with- 

 out resorting to the tedious process of preserving and sectioning 

 the gonads. 



The results of this series of experiments are summarized in 

 Table II. 



