HYBKIDS BETWEEN FUNDULUS AND MACKEREL 399 



envelope, which probably also protects the eggs from injury of 

 various kinds. In correlation with the fact that eggs are ferti- 

 Uzed immediately on their emission from the oviduct, it is note- 

 worthy that the spermatozoa live only a few minutes, at most 

 five, in seawater. Consequently, any females that have been 

 isolated for any appreciable length of time could not possibly 

 carry sperm upon their bodies. Therefore any criticism of these 

 results, based on the assumption that occasional heterogenic 

 hybrids that go through successfully to hatching might be due to 

 chance fertilizations by sperm of the same species, have no 

 foundation in fact. All that one has to do to obviate the possi- 

 bility of any such contingency is to isolate the Fundulus females 

 and handle no Fundulus males during the hybrid experiments. 

 This was done in every case. 



The mackerel, in contrast with Fundulus, is a rather large 

 fish, frequenting the off-shore waters except when they come in 

 closer to breed. They are intolerant of water impurities, die 

 soon in aquaria and show themselves extremely sensitive to lack 

 of oxygen or high CO2 concentration in the seawater. The eggs 

 are about 1 mm. in diameter, typically pelagic in character, but 

 differ from most pelagic eggs in having a distinct pinkish cast. 

 The milt is extremely abundant and is evidently shed in clouds 

 in order to ensure fertilization of the scattered eggs. The 

 spermatozoa live in aquarium seawater for about twenty minutes 

 and probably live even longer in natural seawater. 



Differences between embryos and larvae of the Fundulus and 



mackerel 



The egg of the Fundulus develops much more slowly than 

 does that of the mackerel, taking about two weeks to hatch 

 as compared with about two and a half days for the latter. 

 Only in late larval stages are there significant differences of bod- 

 ily structure and proportions. These do not need description 

 here as none of the conclusions here brought out have to do with 

 such general characters. Our attention must be focused upon 

 one type of character, the peculiarities of the yolk and body 

 chromatophores. 



