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seems not to be the relationship of the sperm, but its vitality and fertilizing 
power. Third, Experiments with various egg extracts and the like on the 
behavior 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. Im all the hybrids here consid- 
ered that succeed in forming a circulatory system at all may begin to de- 
yelop 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, perhaps up to the point of hatching. But in the 
case of some hybrids none of the embryos form a heart and a varying per- 
centage of all hybrids fail to develop the heart at all, even though the 
more successful ones complete development. Furthermore, it often happens 
that the circulatory system is apparently properly established and the de- 
velopment carried to the point of hatching, or even beyond, but they soon 
die. Thus while it is undoubtedly true that the establishment of the cir- 
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