MODES OF INHERITANCE IN HYBRIDS 449 



for a considerable period, in most cases through the cleavage 

 period, in a normal manner. In all cross bred eggs the cleavage 

 seemed to be identical with that of pure bred eggs except that 

 in rare cases the time rate of the cleavage process was markedly 

 retarded. This was especially noticeable in the hybrids from 

 Spinachia eggs fertilized with Gasterosteus sperm. 



An observation of Appellof's, which to my mind is of consid- 

 erable morphological significance, is to the effect that the most 

 critical period in the development of heterogenic hybrids is the 

 period of transition from cleavage to germ layer formation — the 

 period of 'gastrulation.' In comment upon this finding I may 

 say that my own results have for a number of years forced me to 

 similar conclusions and that I have on hand an investigation 

 dealing at some length with this point. It is of more than pass- 

 ing interest to note how this idea lines up with the rather prev- 

 alent opinion that the process of cleavage is purely a function 

 of the egg, and that the spermatozoon, though cooperating with 

 the egg nucleus in the mitoses of cleavage, does not begin to 

 influence the process of heredity until during the formation of 

 the gastrula. It would appear that even a very distantly related 

 sperm may stimulate development in an egg and cooperate har- 

 moniously with the latter through the period of cell multiplica- 

 tion, but that when the hereditary functions of the germinal 

 elements come into play discordant chemical interactions take 

 place that effectually disrupt developmental harmony. This 

 barrier to further development I have found to be less sharply 

 drawn than Appellof supposed, as will become evident when the 

 data subsequently presented in this paper claim the reader's 

 attention. 



Next in chronological order comes the work of Moenkhaus ('94) 

 in which he relates the results of his crosses between Fundulus 

 heteroclitus and Menidia notata. This work had as its object 

 the study of the behavior of maternal and paternal chromatin 

 in hybrid eggs and embryos in which the chromosomes of the 

 parental species were visibly different in form. The observa- 

 tions show that the paternal chromosomes retain their identity 

 in the zygote and for a considerable period remain in a close 



