Heredity in Fundulus Hybrids 151 



Method II. Comparison of all 4's and all 2's. In the pure-bred 

 lot 20.44+ per cent of 4's and 70.69 + per cent of 2's; in the hybrid 

 lot 26.50 + per cent of 4's and 62.89 4- per cent of 2's. There is 

 only a slight difference in the percentage of 3-cell stages in the two 

 strains. 



Method III. Comparison of strict 4's, strict 2's and interme- 

 diates. In the pure-bred lot there are 5.45 + per cent of strict 4's 

 53.15+ per cent of strict 2's and 36.62 + per cent of intermediates; 

 in the hybrid lot, 7.24 + per cent of strict 4's, 50.17 + per cent of 

 strict 2's and 40.45+ per cent of intermediates. The percentage 

 of 2-minus stages is more than twice as large in the pure-bred as in 

 the hybrid strain. 



Experiment ^ 



The eggs of several females divided and fertilized with the milt 

 of two males of each species. Killed four hours after fertilization. 



In this case a surprisingly small number of the pure-bred eggs 

 developed, while a large number of the hybrid eggs cleaved nor- 

 mally. On account of the very small numbers of developing pure- 

 bred eggs only the first m^ethod of comparison will be used. The 

 average number of blastomeres in the pure-bred lot is 2.67 + ; that 

 of the hybrid lot 3.61 + ; showing a marked difference in the same 

 direction as in more successful experiments. 



Experiment 6 



In order to make the comparison between the pure-bred and the 

 hybrid strains more searching, the eggs of one large female were 

 divided into two approximately equal lots and fertilized with the 

 mixed, expressed milt of several males of each species. Careful 

 drawings had previously been made of seven stages between the 

 l-cell and the 4-cell conditions. These were made at approxi- 

 mately equal time intervals and were numbered from i to 7, be- 

 ginning with the earliest stage. All eggs, so far as was possible, 

 were examined and grouped according to their resemblance to the 

 figures. In case an egg falls between two figured stages it is 

 assigned to an intermediate group, e. g., an egg that falls between 



