DIFFERENTIATION, HEREDITY, SEX 303 



position is of the lesser importance in determining the devel- 

 opment of specific traits, and of course seriously affected the 

 validity of the hypothesis of nuclear determination. 



These experiments, however, curiously turned out to afford 

 very strong evidence in support of this hypothesis. For other 

 workers (Herbst, Kupelwieser, Bataillon) showed that in many 

 of these, and in other instances, the nucleus of the spermatozoon 

 did not actually fuse with the egg nucleus, but remained either 

 partly or wholly inactive, taking little or no share in the forma- 

 tion of the mitotic figures of the first and subsequent cleavages 

 (Fig. 140). The resulting larvaB therefore were not truly 

 hybrids; the spermatozoon had merely stimulated the egg to 

 develop, as hi artificial parthenogenesis, but itself took no part 

 in the formation of the nuclear structures of the larva. In the 

 absence of microscopic examination of the embryo, therefore, 

 it is impossible to place any emphasis upon the development 

 of purely maternal or paternal characters under such conditions. 



Fortunately such cytological evidence is now provided 

 extensively through the work "of Baltzer, who has traced the 

 nuclear history of many forms of Echinoderm hybrids. It 

 appears that part or all of the paternal chromatin, never the 

 maternal, may be thrown out of the nuclei of such "hybrids" 

 (pseudohybrids). Such an elimination of paternal chromatin 

 may occur during the very first cleavage, or it may be delayed 

 until the blastula or even early gastrula stage (Fig. 141). The 

 examination of a long series of hybrids, showing all degrees 

 of purity of the maternal characters, leads Baltzer to the con- 

 clusion that the degree to which paternal characters appear in 

 the resulting hybrids, is closely parallel to the relative amount 

 of paternal chromatin which is retained within the nuclei of the 

 organism. Where the fusion of the sperm and egg nuclei 

 remains complete, the hybrids have intermediate characters; 

 where little or no chromatin from the spermatozoon is retained 

 in the nuclei, there appear, chiefly or alone, maternal characters. 

 Only in the case of the fertilization of the eggs of a sea-urchin 

 (Strongylocentrotus) with the sperm of a Crinoid (Antedon) has 

 it been shown that the fusion of the germ nuclei really occurred 



