34 HELEN DEAN KING. 



the eggs were laid fully one week earlier than is the usual breed- 

 ing time for toads in eastern Pennsylvania. Of the 65 I individ- 

 uals in which sex was ascertained 359, or 55.14 per cent., were 

 females. This result does not accord with Hertwig's view that 

 the younger the egg at the time of its fertilization the greater its 

 tendency towards male production. No eggs were obtained in 

 the spring of 1907 from the end of March until the last week in 

 April when several pairs of toads were brought into the laboratory 

 and a large number of eggs laid and normally fertilized. Ac- 

 cording to Hertwig's theory these eggs were all overripe and 

 should, therefore, have given a preponderance of males. To this 

 series belong the eggs used in the experiment described in sec- 

 tion I. The results of the experiment in question, which are 

 summarized in Table I., show that in the total of 600 individuals 

 in which sex was ascertained there were 341 or 56.83 per cent, 

 of females. This is about the proportion of females that is ap- 

 parently normal for the species under natural conditions, and 

 this normal sex ratio does not seem to be altered whether the 

 eggs are laid the latter part of March or retained by the female 

 until the end of April. 



The second experiment that was made to test the influence of 

 the ripeness of the egg at the time of fertilization on sex-deter- 

 mination was carried out as follows : A female which was isolated 

 on the afternoon of April 5, 1908, began laying at 2 A. M. the 

 following morning. She was at once killed by pithing and the 

 body, unopened, placed in a moist chamber. At 9 A. M., seven 

 hours later, the eggs were removed from the body of the female 

 and artificially fertilized, sperm from two males being used. Al- 

 though the eggs appeared perfectly good many of them did not 

 fertilize, and the segmentation of the fertilized eggs was slow and 

 in many cases abnormal. It is evident, from this fact, that post- 

 mortem changes had already begun in the eggs at the time that 

 they were fertilized and that it would not be possible to obtain 

 normal embryos from eggs of Bufo that remained unfertilized 

 much longer than seven hours after the death of the female. Al- 

 though the eggs in this series were placed under the most favor- 

 able conditions possible many of them failed to gastrulate and 

 the tadpoles that lived developed very slowly, large numbers of 



