Hybridization of Echinoids. 35 



acutus treated by hypertonic solutions. The results were similar. 

 There was an elimination of a certain number of chromosomes from 

 the nucleus in both the cross-activated eggs and in the straight-fer- 

 tilized eggs which had been treated with hypertonic solutions. The 

 eggs of Echinus esculentus were not affected by similar treatment. 



Gray suggests, tentatively, after consideration of the results of 

 McClendon (1913) and R. S. Lillie (1909, 1911), on increased perme- 

 ability of the egg membrane after the entrance of the spermatozoon, 

 that the degree of change in permeability after fertihzation is a func- 

 tion of the sperm and that the cytological behavior of reciprocal 

 crosses is explicable on this hypothesis. Gray points out that changes 

 in permeability of the cytoplasm as induced by the sperm can not 

 be a sufficient explanation of all abnormalities observed in cross- 

 fertilized eggs. 



Doncaster and Gray (1913), in considering the probable fact that 

 the vesicles in the eggs of the cross acutus X esculentus are derived 

 from the acutus chromosomes, suggest that the explanation may lie 

 in an alteration of the permeability or osmotic condition of the egg, 

 consequent upon the development within it of a foreign spermatozoon, 

 i. e., the physical condition of the cytoplasm is altered by the develop- 

 ment within it of a foreign sperm nucleus. 



Nuclear Enzymes and Cytoplasmic Substrate. 



The study of a sufficient number of species, both closely related 

 and widely separated, has now given, I believe, an adequate basis for 

 generalization. 



The egg contains the mechanism necessary for development. It 

 may develop parthenogenetically, giving a maternal larva and adult. 

 In that development there is a harmonious interaction between the 

 nuclear and cytoplasmic material. The results of cross-activation 

 in which there is a complete rejection of paternal nuclear material 

 and production of false hybrids, as in the Echinoid-Mollusk, or Echi- 

 noid-Annelid cross, are closely related to examples of artificial par- 

 thenogenesis. 



Above this level there are crosses showing maternal or paternal 

 influence in lesser or greater degree. We might pass in review a 

 great number of cases of^ partial hybridization and reach finally 

 undoubted examples of true hybridization. 



If we restate these facts on the basis of chromatin content, we find 

 that in false hybrids there is a complete elimination of paternal 

 chromosomes, in partial hybrids a partial elimination of chromosomes, 

 and in true hybrids no elimination of chromosomes. 



The fact that the results obtained from a given cross differ from 

 those obtained in the reciprocal cross indicates that the differences 

 are not due to lack of harmony between the substances of the germ 



