388 



be accounted for by the abuormally large pericardial cavity which de- 

 velops, across which it becomes stretched. The large pericardial cavity may 

 be the result of the abnormal method of yolk absorption due to the failure 

 of the blood vessels to differentiate. 



I have for three or four years looked upon those iihenomena in my 

 own hj'brid experiments as a process akin to that which obtains in the 

 transfusion of blood of strange species. The well known results of Lan- 

 dois ('75), Friedenthal ("99) and others bring out the important fact that 

 the htemolytic po\^'er of the bloods of U\o species varies in intensity with 

 the nearness of relationship of the species. In general two very closely 

 related animals will permit the transfusion of their bloods with no or rela- 

 tively slight ligemoij'tic action. As the forms become divergent in rela- 

 tionship the toxic action becomes progressively greater. In a similar man- 

 ner it has been shown that other tissues than blood act toxically. Among 

 these are spermatozoa. The process in hybridization may be conceived 

 something as follows : ^\'he]i the sperm brings its material into the strange 

 egg in fertilization it brings with it the substance capable of poisoning the 

 egg substance or vice versa. We may suppose that the toxic action does 

 not manifest itself at once because of the relatively small proportion of the 

 sperm substance compared to that of the egg. Consequently early cleavage 

 stages are in all cases passed through in a normal manner. As, however, 

 the nuclear material grows and becomes more generally distributed through 

 the cytoplasmic mass as cleavage proceeds, the toxic action becomes mani- 

 fest in the retardation of the cleavage and subsequent developmental pro- 

 cesses. The intensity of the effect will vary with the degree of toxidity 

 existing between the two species concerned. In the cases of fishes where 

 cross fertilization is so generaluy possible it should be possible to get a 

 measure of this in the faithfulness with which the embryo reproduces the 

 normal developmental processes in the earlier stages, and the stage at 

 which these become arrested. 



In the transfusion of bloods we have seen that the toxidity varies 

 rather closely with the systematic relationship of the animals. My ex- 

 periments so far as they go. show that this same law holds in hybridiza- 

 tion, and when taken in connection with what is already well known 

 about the production of so-called "successful"' hybrids, I think, may be 

 interpreted as furnishing evidence for this view. 



