462 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the rate of development is accelerated, while in those between distantly 

 related species development i& retarded, but not necessarily in direct 

 proportion to the heterogeneity of the cross. Although foreign sperm 

 may materially alter the rate of early development, it plays no role in 

 the heredity of the organism until embryonic differentiation is well 

 under way. 



A study of the inheritance of pigment in hybrids of species of 

 Fundulm leads to the conclusion that all the well-known modes of 

 inheritance are illustrated — that characterized by dominance or even 

 hyperdominance, that commonly called blended or intermediate, and 

 that usually called mosaic or particulate. There is another mode 

 distinct from any of these. It is still an open question whether these 

 types could be successfully reduced to an orthodox Mendelian basis, 

 were the unit factors involved all analysed out. 



It seems as if any Teleost spermatozoon might play a role in cleavage 

 equivalent to that of agents that are successful in artificial partheno- 

 genesis, but that only certain kinds of sperm, that can successfully 

 co-operate with the egg-nucleus in its hereditary activities, are capable 

 of working out a complete ontogeny. 



The idea expressed by Moenkhaus that success in the development 

 of Teleost crosses is a function of the phylogenetic relationship of the 

 parents cannot be upheld. For there is a marked difference between 

 the developmental success of reciprocal hybrids, and success in the 

 development of hybrids produced by crossing different orders of 

 Teleosts is not seldom greater than that attained by hybrids between 

 different families of the same order, or even between different genera 

 of the same family. 



Hybridizing Teleosts.* — H. H. Newman has made ninety-three 

 crosses among fourteen species of Woods Hole Teleosts — species of 

 Fundulus, Cyprinodon, Gaslerotteus, Scomber, Tautoga, and other genera. 

 He discusses the heterogenic crosses between members of different 

 orders or families. In seventeen cases he got some embryos with a 

 circulation : in sixteen cases he got larvfe. There is as much develop- 

 mental success among the heterogenic crosses as among some homogenic 

 crosses (between different genera of a family or species of a genus). In 

 general, there seems to be no primary correlation between the degree of 

 success in development and the nearness of relationship of the species 

 cross. The factors determining the relative success in the development 

 of hybrids are associated with the amount, density, and specific com- 

 position of the yolk and with the hardiness of the egg. The rate of 

 development seems unimportant. 



In many cases definite paternal characters are seen in the hybrid, 

 which goes to disprove Loeb's view that the sperm in heterogenic 

 hybridization plays only the role of initiating development. But the 

 most successful embryos that develop to hatching are predominantly 

 maternal, perhaps only apparently so. Monsters are common, all of them 

 interpretable as due to retardation of development through the dis- 

 harmonious interaction of foreign germ-plasms. The main blocks occur 



* Journ. Exper. Zool., xviii. (1915) pp. 511-76 (11 figs.). 



