54 The Chromatin in the Development of Hybrids 



These objections have largely been disposed of by the researches of 

 Ruckert, Hacker, Herla, Conklin and others. These leave little doubt 

 that the maternal and paternal chromosomes may remain distinct to a 

 late stage in development, and I have shown that however thoroughly 

 the chromosomes may lose their identity to our view during the resting 

 period of the cell they nevertheless retain their individuality. 



In view of its possible bearing on the theories of heredity just now 

 becoming prominent through the recent rediscovery of the Mendelian 

 laws of inheritance, it is highly desirable that this question of the 

 individuality of the parental chromosomes be most thoroughly investi- 

 gated. Further observations along this line on other hybrid fishes I 

 have well under way, which I hope to be able to present in the near 

 future. 



SUMMARY. 



The eggs of Fundulus heteroclitus can be readily impregnated with 

 the sperm of Menidia notata. From 70 to 93 per cent of the eggs are 

 fertilized. Of this number about 50 per cent are dispemiic, the re- 

 mainder, normal. 



The eggs of Menidia notata can be even more completely impreg- 

 nated by the sperm of Fundulus heteroclitus. Under favorable circum- 

 stances 96 per cent of the eggs are fertilized. Of these only a few are 

 dispermic or polyspermic. 



The normally impregnated eggs of both crosses develop normally to 

 varying stages of embryo formation. They never go beyond the closure 

 of the " blastopore." 



The embryos differentiate the three germ layers, the chorda and neural 

 cord. In rare instances the eyes may begin to develop — the optic cup 

 and the lens being formed. 



The per cent of eggs that develop to the closure of the " blastopore " is 

 comparatively small. The per cent is much greater in the Fundulus 

 hybrids than in the reciprocals. 



The more usual thing is for the embryos to show abnormalities. These 

 appear during the process of gastrulation and are probably all the ex- 

 pression of a weakening of the developmental energy. 



The abnormalities take the form of variously shortened embryos with 

 the " blastopore " completely closed or imperfectly so, in which case the 

 latter may take the form of a long slit or of a cleft of varying irregu- 

 larity in shape. 



The early cleavage stages are passed through in a perfectly normal 

 manner. The blastomeres show no greater variation in form from the 

 typical than do normal eggs. 



