344< The Unity of the Organism 



weighty as evidence that chromatin plays a very important 

 part of some sort in heredity. 



Fertilization of the Ova of One Species hy the Sperm of 



Another Species 



The class of facts next to be noticed as supporting the 

 chromosome theory of heredity has come to light through ex- 

 perimental researches, and concerns the cytological results 

 of fertilizing the eggs of one species Avith the sperm of 

 another. During the last fifteen years considerable work of 

 this sort has been done. Most of it has produced equivocal 

 results, but some of the positive results favor the chromo- 

 some theory, while others oppose it. At present we will 

 consider only those which favor it. Boveri, one of the most 

 intellectually resourceful and manually deft investigators in 

 this as in so many other aspects of cytology, writes : "The 

 egg protoplasm is with reference to these qualities [i.e., of 

 individual and species] only the material for the forma- 

 tive activity of the equally potent but opposite male and 

 female nuclear parts." ^^ Although this formulation was 

 made with special reference to Boveri's own observations, 

 as a matter of fact the e\adence which seems to support 

 it most strongly has been produced not by Boveri but by 

 Curt Herbst. Herbst's most telling case is presented in 

 his Studies in Heredity. The evidence obtained "is 

 almost convincing, I think," says Morgan, "in favor of 

 the view that chromosomes are the essential bearers of 

 the hereditary qualities." ^^ 



Herbst conceived the interesting experiment of giving 

 spermatozoa a chance at eggs which had already received 

 the impulse to develop without the intervention of sperm, 

 that is, parthenogenetically.^^ By using the eggs of one 

 species and the sperm of another, he thought he might be 

 able to recognize the difference in effect of the female and 



