MODES OF INHERITANCE IN HYBRIDS 457 



the two species used? If not, upon what factors does success 

 in crossbreeding depend? 



6. Are structural or physiological characters inherited in the 

 exclusive or in the blended fashion in the first generation of 

 hybrids? 



7. \\Tiat is the function of the spermatozoon in early develop- 

 ment? Is its initial effect merely equivalent to that exercised 

 by mechanical and chemical agents used in artificial partheno- 

 genesis, or is this only part of its function? 



8. Is there any truth in the analogy suggested by Moenkhaus 

 between the effects resulting from mixing strange sex products 

 and those resulting from the transfusion of unrelated bloods? 



Each of these questions will receive its proper share of atten- 

 tion elsewhere in the paper. 



NEW EXPERIMENTS: RECIPROCAL CROSSES BETWEEN SPECIES OF 

 THE GENUS FUNDULUS AND THE GENUS CYPRINODON 



1. MATERIALS AND METHODS 



The four species of fish used in the present experiments belong 

 to two genera of the family Poeciliidae and are common in the 

 waters about Woods Hole. They are Fundulus heteroclitus (figs. 

 1 and 2), F. majalis (figs. 3 and 4), F. diaphanus (figs. 5 and 6), 

 and Cyprinodon variegatus (figs. 7 and 8). F. heteroclitus, F. 

 majalis and Cyprinodon are marine or brackish water species, 

 while F. diaphanus is strictly a fresh water form though tolerant 

 of slightly brackish water. In some regions F. heteroclitus is 

 found with F. diaphanus in fresh water ponds into which the 

 sea flows at high tide. The eggs of F. majalis are the largest, 

 averaging about 2.7 mm. in diameter, those of F. diaphanus come 

 next with an average diameter of 2.3 mm., those of F. heteroclitus 

 average 2 mm., and those of Cyprinodon are much the smallest, 

 averaging scarcely 1.5 mm. The yolk of F. majalis eggs is denser 

 and of deeper yellow color than that of the other species; that 

 of F. heteroclitus is denser and yellower than in F. diaphanus 

 but less so than in F. majalis; that of Cyprinodon is almost 

 colorless. 



