The Process of Heredity 517 



III DESCRIPTION OF EXPERIMENTS AND PRESENTATION OF DATA 



Data Derived from the Study of Living Material 

 I. Type Series (Series i) (Plates II, III and IV) 



This series was started at 2 p.m., on July 2, 1907. Fresh lots 

 of fish of both species were brought in and the conditions for the 

 experiment were practically ideal. 



The stages of development earlier than those presented in the 

 table, as well as those later than the seven day period (the last 

 stage pictured in the table), will be described verbally or with the 

 aid of occasional isolated illustrations. 



Conditions earlier than Stage i {18 hours), a Comparative 

 fertility of eggs to sperm of their own and that of the foreign species. 



Out of 121 eggs of// pure, 108 cleaved (89 per cent fertile). 

 Out of 136 eggs of H hybrid, 84 cleaved (61 per cent 



fertile). 

 Out of 92 eggs of M pure, 82 cleaved (88 per cent fertile). 

 Out of 103 eggs of M hybrid, 57 cleaved (45 per cent 



fertile). 



In both cases the eggs were more fertile to sperm of their own 

 than to those of foreign species. 



h Comparative rates of cleavage of pure and hybrid strains. 

 After two hours nearly all of the // pures and the // hybrids had 

 cleaved and were in the two-cell stage. About twenty minutes 

 later over half of the eggs in these two strains were in the four- 

 cell stage. Rapid counts of blastomeres of stages up to the sixteen- 

 cell stage convinced me that there was no appreciable difference 

 in the rates of early cleavage in pure and hybrid strains. Three 

 hours after fertilization (one hour later than in the heteroclitus 

 strains) both majalis strains had cleaved and were in the two- 

 cell stage. They also showed equal rapidity in passing to the 

 four, eight, sixteen-cell stages. 



Other stages up to Stage i (18 hours) were passed over in the 

 night. No observations were made during this period. 



Stage I (18 hours) (see Plate II): Ocular micrometer measure- 



