MODES OF INHERITANCE IN HYBRIDS 471 



of development there is a large external yolk sac full of undigested 

 yolk. In fish the development of the pericardium, heart and 

 vitelline circulation are intimately associated, and under normal 

 conditions the yolk sac diminishes in size at such a rate that it, 

 together with the pericardium and heart, are simultaneously 

 drawn into the body cavity of the embryo shortly after hatch- 

 ing. In these abnormal hybrids, however, the differentiation of 

 the heart and pericardium is at first accelerated, while yolk di- 

 gestion goes more slowly than in pure bred embryos. Since the 

 three structures are intimately associated, this failure of the yolk 

 sac to diminish causes the pericardium to enlarge and stretches 

 the heart into a long straight tube which beats feebly but carries, 

 in later stages at least, no blood. Thus the supply of nutriment 

 is cut off and growth ceases. Various organs continue to differ- 

 entiate even without an external food supply, so that we may 

 have an embryo developed which, though much smaller than the 

 normal, has reached a stage of advancement equivalent to that 

 seen in young larvae of pure bred strains. In last analysis the 

 stoppage of development seems to be conditioned by a lack of 

 coordination between two processes, that of the differentiation 

 of the protoplasmic materials, which is accelerated by certain 

 agents brought by the foreign sperm, and that of yolk assimila- 

 tion which fails to progress as rapidly as in normal eggs. The 

 result is that the embryo gets to the point when heart pericar- 

 dium and yolk sac should be taken into the body cavity and is 

 prevented from so doing by the large mass of undigested yolk. 

 The yolk of F. majalis is optically denser and of different color 

 from that of the other species and it may well be that a specific 

 enzyme carried by the sperm of the same species is necessary for 

 its complete dissociation and assimilation. 



When I say that these abnormal embryos, burdened as they 

 are with a permanent yolk sac, never hatch, I do not mean that 

 they may not be shaken or dissected out of their membranes. 

 AMien Bancroft claims that these embryos occasionally hatch I 

 judge that he means that they msiy sometimes lose the egg mem- 

 brane. Real hatching, however, is brought about by well marked 

 violent struggling of the larva, and no such hatching struggles 



