379 
The low percentage of impregnation, on the other hand, must be re- 
garded in most cases, I feel sure, as due to unfavorable conditions of the 
milt, and in some cases to the unripe condition of the eggs. Males that 
have passed the height of their breeding season, or which may have been 
less able to endure the conditions of confinement in aquaria usually show a 
reduced fertilizing power compared to perfectly fresh and ripe individual. 
The testes were in all the experiments cut out, so that it is quite probable 
that in many cases imperfect milt was used. I was, furthermore, not able to 
establish any constant difference in the percentage of impregnations in re- 
ciprocals. Allowing for the influence of the condition of the milt in deter- 
mining the percentage of impregnation, in all cases where a fair trial was 
made in reciprocal crossing of two species it was approximately as high in 
one direction as the other. It is interesting to note here that Kammerer 
‘OT using fresh water fishes, found, :mong the few forms he used, two crosses, 
Perea fluviatilis x Acerina schraetser and Lucioperea sandra x Perea fiuvia- 
tilis, in which it was possible to impregnate when the first named in each 
case was the male, but not if female. It is also impossible to fertilize the 
eggs of Aspro zingel with the milt from the following nearly related forms: 
Perca fluviatilis, Lucioperca sandra and Acerina sp?, but was able to fer- 
tilize them with the milt from the distantly related form Cottus gobio. It 
would seem from these experiments that fresh water fishes lend themselves 
less generally to hybridization than the marine species. 
Kammerer’s statement that the eggs of Aspro zingel are fertile to the 
sperm of the distantly related form Cottus gobio when they were immune 
to the three nearly related forms above indicated, because Cottus had ia 
similar habitat, and had with this also acquired the power to fertilize this 
species is, of course, a mere fancy. If he had tried to cross this form with 
other distantly related forms he would probably have found that they, too, 
would fertilize the eggs regardless of their habitat relationship. 
DEVELOPMENT. 
In my study of the development of these yarious hybrids I have not 
attempted to get a complete morphological picture, nor have I paid much 
attention to the inheritance aspect. I have regarded development rather 
from a physiological standpoint. The main points of interest, therefore, 
have been, first, how generally and within what limits can the sex-products 
of the various forms of teleosts be grafted upon each other, so to speak, 
