HEREDITY IN HETEROGENEOUS HYBRIDS £> 



and possibly one egg in a million reaches the pluteus stage. The 

 development of the pluteus is in such cases always retarded. 



We find such a retardation not only in the case of heterogeneous 

 hybridization but occasionally also in the case of crosses between 

 closely related forms. While the hybrid purpuratus 9 , francis- 

 canusc? is vigorous, the hybrid franciscanus 9 , purpuratus cT, is 

 sickly and reaches the pluteus stage only rarely and slowly. 



The rate of development of the embryo is a function of the 

 velocity of certain chemical processes which are linked together 

 like the wheels in a mechanical machine. If one of such processes 

 be retarded the rate of development of the whole embryo is likely 

 to be retarded; and if the hnkage of the various chemical proc- 

 esses become disturbed the embryo is likely to be sickly. The fur- 

 ther apart the species are from which the two sex cells originate 

 the greater the likehhood that the rate of development is retarded 

 and that the hybrid embryo is sickly. Why is the development 

 not retarded from the beginning? Possibly for the reason that 

 it requires some time before the spermatozoon can cause the 

 formation of a sufficient amount of harmful substances to cause a 

 retardation of the development of the egg. 



4. Moenkhaus found that the eggs of bony fishes can be easily 

 impregnated with foreign sperm but that they do not develop 

 very far. Thus he states, that the hybrids between Menidia 

 and Fundulus heteroclitus ''never go beyond the closure of the 

 blastopore." I have been able to raise the hybrid between 

 Fundulus heterochtus 9 and Menidia, Ctenolabrus and Stenotomus 

 cf in large numbers beyond this stage. These hybrids lived a 

 month or longer, formed hearts, blood vessels, eyes, and fins but 

 never hatched. With a few exceptions no circulation was ever 

 established although the heart beat for weeks. 



Figs. 14 to 16 show three different hybrids of Fundulus hetero- 

 clitus 9 and Menidia o^, from a lot fertihzed the 12th of June. 

 The camera drawings were made from the living material the 2d, 

 3d and 12th of July. At that time the pure breed of Fundulus 

 heterochtus fertilized on the same date were already hatching. 

 The hybrid embryos had formed the pigment characteristic for 

 the pure breed of Fundulus heteroclitus. But the anomalies of 



