388 
be accounted for by the abnormally 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 these phenomena in my 
own hybrid 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 faet that 
the hvemolytic power of the bloods of two 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 hzmolytic 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: When 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. 
