506 H. H. Neiu??ian 



objects within an envelope, needing no especial care and provided 

 with their own food. 



9 The egg membrane of both species is a thick, resistant 

 capsule, capable of protecting the developing embryos from the 

 various detrimental factors of their environment and making it 

 more nearly possible to study heredity uninfluenced by extraneous 

 phenomena. 



10 Many other special advantages that apply only to the two 

 species used will receive notice in the body of the paper. 



II MATERIAL AND METHODS 



Materials — A Descriptive Account of the Species Used 

 Fundulus majalis and Fundulus heteroclitus, two species of 

 killifish belonging to the family Poeciliidae, furnished the material 

 for the breeding experiments detailed below. Both species abound 

 along the Atlantic coast and are familiar to all workers at the 

 Woods Hole Marine Biological Laboratories, where this work 

 was done. 



These two species interbreed readily and one of the reciprocal 

 crosses is capable of hatching and of living in aquaria at least 

 seven months after hatching. The two species present well 

 marked differentiating characters in adults, embryos and eggs, 

 both morphologically and physiologically. An account of these 

 differences will furnish a logical introduction to the body of the 

 work. 



I. Morphological Differences Between the Adults 



A reference to Plate I will serve to show the strikingly different 

 general appearance of the two species. The actual average size 

 of mature adults is represented in the figures and it will be readily 

 seen that F. majalis is the larger though more slender species. A 

 pronounced sexual dimorphism is exhibited in both species. 

 This phenomenon is concerned chiefly with the shape and size 

 of the fins, color pattern, degree of pigmentation, the presence in the 

 males during the breeding season of minute "contact organs" that 

 are of use in clasping the female, etc. Although this sexual 



