HEREDITY IN HETEROGENEOUS HYBRIDS 9 



chromosomes of the sperm are thrown out of the egg or dis- 

 integrate. This is not in harmony with the observations of 

 Moenkhaus for the hybrids between Menidia d^ and Fmidulus 

 heterochtus 9 , and with those of Godlewski for the hybrids 

 between ^ea-urchin and crinoid. I am not in a position to decide 

 the differences in the observations of these authors. The obser- 

 vations mentioned in the preceding paragraph are more in 

 harmony with the observations of Moenkhaus and Godlewski. 



CONCLUSIONS 



The spermatozoon has two distinct effects upon the egg: namely, 

 it causes its development and it transmits certain parental hered- 

 itary characters to the offspring. The experiments in heter- 

 ogeneous hybridization confirm the idea supported by the exper- 

 iments on artificial parthenogenesis, that the formation of the 

 embryo is purely a matter of the egg and that the main function 

 of the spermatozoon is the causation of the development of the 

 egg. If we may express this statement in the form of a paradox 

 we may say that fertilization is primarily and essentially artifi- 

 cial parthenogenesis. The transmission of hereditary characters 

 through the sperm is in many cases merely an accessory function. 

 It becomes of vital importance only in those forms where the male 

 is heterozygous for sex and where the species can only be propa- 

 gated through sexual reproduction. 



If the sperm nucleus be chemically almost identical with the 

 egg nucleus it is- possible for it to force one or a few characters 

 upon the developing embryo. If the difference between sperm and 

 egg nucleus exceed a certain limit — which structural chemistry 

 may one day be able to define— the hereditary influence of the 

 spermatozoon is as a rule completely or almost completely oblit- 

 erated; and the result is a purely maternal larva, rendered more 

 or less sickly through the presence or formation of foreign or 

 faulty substances. 



The camera drawings of the sea-urchin larvae were made by 

 Mr. W. 0. R. King, those of the fish embryos by Mr. Bagg. To 

 both gentlemen I wish to express my thanks. 



