DEVELOPMENT IN CROSS-FERTILIZED EGGS 413 



2. Chromosome behavior and taxonomic relationships 



Whatever the nature of the physical condition governing the 

 mitotic processes at the critical anaphase stage, the condition 

 itself is no doubt an expression of the specific chemical composi- 

 tion of the egg cytoplasm. This chemical composition may 

 be more highly specific than the physical state which it condi- 

 tions, that is, it is conceivable that the egg protoplasm of two 

 species of fish may differ chemically and yet resemble each other 

 so closely in their physical characters that they may react alike 

 in crossing. In other words, protoplasmic relationships between 

 species are not necessarily correlated with the physical condi- 

 tions present in their germ cells at corresponding morphologi- 

 cal stages. This, I believe, explains the fact that the results 

 of heterogeneric hybridization show no correlation with taxo- 

 nomic relationships. 



While the presence or absence of abnormal mitosis in early 

 cleavage is not correlated with taxonomic relationships, a com- 

 parison of the crosses made shows some indication of an under- 

 lying relationship based on the egg's behavior in this respect 

 which is independent of species afF.nities. For instance, refer- 

 ence to the tabular summary above shows that when the germ 

 cells of Menidia and Fundulus combine reciprocally, develop- 

 ment is not hindered by this block to normal mitosis, although 

 it may, and usually does, meet with some disturbing factor later 

 on. Both of these eggs exhibit such a block to the sperm of 

 Ctenolabrus; that is, both show the same behavior to the same 

 foreign sperm. Further, the reciprocal crosses of Ctenolabrus 

 and Prionotus show similar behavior in that both proceed nor- 

 mally. The egg of Fundulus produces the same reaction in the 

 sperm of both of these species. I should hke to have made fur- 

 ther tests by crossing Menidia 9 with both Prionotus cf and 

 Stenotomus cf, but unfortunately Menidia eggs were not ob- 

 tained in 1921. One of course hesitates to draw conclusions 

 from so little evidence, but the facts are certainly significant. 



A review of the reactions of echinoderm eggs in hybridiza- 

 tion reveals certain similarities of behavior which favors the 



