Chemical Basis of Genus and Species 51 



with the development of a certain type of wandering 

 cells, the mesenchyme cells; it would perhaps be worth 

 while to investigate this possibility. The writer was 

 under the impression that this sickliness might have 

 been brought about by a poison gradually formed in 

 the heterogeneous larvse. 



He investigated the effects of heterogeneous hybridi- 

 zation also in fishes, which are a much more favourable 

 object. The egg of the marine fish Fundulus hetero- 

 clitus can be fertilized with the sperm of almost any 

 other teleost fish, as Moenkhaus' first observed. 

 This author did not succeed in keeping the hybrids 

 alive more than a day, but the writer has kept many 

 heterogeneous hybrids alive for a month or longer,^ 

 and found the same two striking facts which he had 

 already observed in the heterogeneous cross between 

 sea urchin and starfish: first, practically no transmis- 

 sion of paternal characters, and second, a sickly con- 

 dition of the embryo which begins early and which 

 increases with further development. The heterogene- 

 ous fish hybrids between, e. g., Fundulus heteroclitiis 9 

 and Menidia & have usually no circulation of blood, 

 although the heart is formed and beats and blood- 

 vessels and blood cells are formed; the eyes are often 

 incomplete or abnormal though they may be normal 

 at first ; the growth of the embryo is mostly retarded. 



' Moenkhaus, W. J., Am. Jour. Anat., 1904, iii., 29. 

 ' Loeb, J., Jour. MorphoL, ic}i2, xxiii., i. 



