HYBRIDS BETWEEN FUNDULUS AND MACKEREL 397 



Haplomi, and Scomber scombrus of the suborder Acanthop- 

 terygii. This cross between mud-minnow and mackerel is an 

 excellent one for our purposes because it shows in the clearest 

 way what many other crosses show, but show in a less obvious 

 way. 



One reason for making an intensive study of one particular 

 heterogenic cross was because I have felt that certain incorrect 

 conclusions as to the nature of heterogenic Teleost hybrids had 

 been published by well-known writers. Moenkhaus holds that 

 the development of these hybrids is pure maternal and that 

 the cleavage rate of the hybrid egg is unaltered by the foreign 

 sperm even if the cleavage rate of the paternal species is much 

 slower or much more rapid than that of the maternal species. 

 I have shown that this is incorrect by the use of more exact meth- 

 ods of comparison. There is an acceleration of cleavage and 

 development, accompanied by a heightened vigor when two very 

 closely allied species are crossed. In any crosses except those of 

 very closely allied species the rate of cleavage and subsequent 

 development is retarded and more or less abnormal (subnormal) 

 larvae result. This does not depend directly on the distance of 

 the cross, for sometimes crosses of suborder width shpw less 

 retardation and a higher viability than crosses of generic width. 



Loeb, on the basis of certain rather unfortunately chosen 

 crosses, supports Moenkhaus in his statement that cleavage rate 

 and that of subsequent development is pure maternal and goes 

 farther in claiming that heterogenic hybrids are pure maternal 

 in their hereditary characters. He thinks the hybrid larvae, 

 which in all of his experiments appeared to be quite unhealthy, 

 are merely pure maternal or parthenogenetic larvae that are 

 poisoned by the presence of materials brought in by the sperm. 

 The argument in favor of the pure maternal character of devel- 

 opment, which implies parthenogenesis, is that ''if the develop- 

 ment of the egg were caused by an enzyme carried into the egg 

 by a spermatozoon, developing eggs should be accelerated by 

 a spermatozoon of a species developing at a faster rate." This 

 conclusion does not appear to me to follow at all, for we must 

 not forget the high specificity of enzymes. An enzyme that is ca- 

 pable of setting up a high rate of activity in one species of proto- 



