385 



seems not to be the relationship of the sperm, but its vitality and fertilizing 

 power. Third, Experiments with various egg extracts and the lilie on the 

 beliavior of spermatozoa give no evidence of any attraction of an egg 

 for its own sperm or any toxic influence upon the strange sperm. It 

 seems, therefore, that in the case of these teleosts there is no evidence of 

 any specific adaptation of the egg for its own spermatozoon. 



How can we account for these varying degrees of failure in develop- 

 ment in these various hybrids? This question is as old as our knowledge 

 of the common infertility of hybrids. Why should an animal or plant 

 hybrid carry its development in a perfectly normal and healthy manner 

 up to the final stage of sex product formation, and yet at this point so com- 

 monly fail? To this question we have up to the present time no definite 

 answer whatsoever. 



DEGREE OF DEVELOPMENT AND SYSTEMATIC RELATIONSHIP. 



In following the development of the various hybrids hereunder discussed 

 there appears one period in the development *to which we might ascribe 

 the failure of development, more than any other: this is the defective devel- 

 opment of the circulatory system. Development in most crosses proceeds 

 often in a relatively normal manner up to the period of the differentiation 

 of the heart, blood vessels and the blood. In all the hybrids here consid- 

 ered that succeed in forming a circulatory system at all may begin to de- 

 velop the heart more or less normally, so that it regularly and vigorously 

 pulsates but fails to differentiate the blood and blood vessels. As a result 

 the heart manipulates no normal blood and, as a consequence, the food ab- 

 sorption of the embryo must occur through other channels than the blood. 

 Following this period the embryos invariably begin to lag behind, the organs 

 fail to properly differentiate, resulting in the stunted, sickly-looking, 

 starved hybrid. It would seem that if it were possible in some way to 

 help the hybrids to properly complete this system, development might be 

 carried much further. i>erhaps up to the point of hatching. But in the 

 case of some hybrids none of tlie emJn-yos foi-m .-i heart and a varying per 

 centage of all liyl)rids fail to develop the heart at all. oven though the 

 more successful ones complete develo])inent. Furthermore, it often happens 

 that the circulatory system is apparently properly estal^lished and the de- 

 velopment carried to the point of hatching, or even beyond, but they soon 

 die. Thus while it is undoulitedly true that the establishment of the cir- 



[25—26988] 



