391 
tail, they form no eyes. Occasionally one or two poorly developed ear 
vesicles show; pigment is irregularly and rather sparingly developed on 
both the embryo and the yolk. These cells are practically all black with a 
few small, poorly developed brownish ones. The heart may develop into a 
protoplasmic pulsating mass showing no definite form. The pericardial 
cavity is poorly developed or wanting. These embryos may remain alive 
for a week or ten days, but never as long as the hybrids of the two pre- 
ceding groups. 
We can see from the foregoing that within the narrow limits of the 
species covered, that the more distantly two species are separated in 
their blood relationship when crossed, the earlier the developmental pro- 
cesses come te a standstill. The writer, of course, thoroughly appreciates 
that the foregoing facts are not necessarily evidence in favor of the view 
taken. He desires merely to emphasize the analogy existing between the 
conditions of hybridization and the known conditions of blood transfusion 
and the like. His belief that this analogy is a significant one has been 
strong enough to lead him into further, more direct experiments along this 
line. The writer may even be permitted to express a hope of his that it 
may be possible to control the processes of hybridization in a manner sim- 
ilar to that which has already been brought about in the field of immunity. 
SUMMARY. 
1. The eggs of any of the species of teleosts tried may be impreg- 
nated by the sperm of any other species tried. 
2. The number of eggs fertilized is usually great, i. e., 75% or more. 
This bears no relation to the nearness of relationship of the two species 
concerned. 
3. Normal impregnation is the rule, di- and polyspermy being the 
exception. 
4. Development in its early stages proceeds normally, the deleterious 
effects of the two strange sex products upon each other showing only at 
later cleavage or subsequently. 
5. The rate of development in the early cleavage stages is always 
that of the egg species. Any effect of the strange sperm upon the rate of 
development shows itself by slowing the process regardless of whether the 
rate of the sperm species is faster or slower than the egg species. 
6. A period of great mortality in the developing hybrids is gastrula- 
