MODES OF INHERITANCE IN HYBRIDS 



465 



TABLE 6 

 Hybrids between Cyprinodon variegatus 9 X F. diaphanus J' 



2 hours 



6 hours 



20 hours 



26 hours 



2 days 



3 (lays 



4 (lays 



About 75 per cent of eggs cleaving an(i 4- to 8-cell stages. Cleavage 

 regular without any in(iications of polyspermy 



All fertilized eggs in advanced cleavage stages, like control but 

 less advanced 



A few of the eggs that had cleaved are dead and part'ally disinte- 

 grated, but the majority have reached the end of the cleavage 

 period and are beginning to show evidences of gastrulatioru All 

 are decidedly less advanced than control, in which all eggs are 

 in germ ring stages 



Nearly half of the embryos that had reached the stage described 

 above now show cytolysis, the others are still alive and in early 

 germ ring stages 



Only four embryos still alive. These have the germ ring one-third 

 to one-half around the yolk, a triangular embryonic shield and 

 a well-defined embryonic axis. Many embryos have died in early 

 germ ring stages 



Two embryos alive. These have blastopore nearly closed but show 

 very little embryonic differentiation 



Both embryos dead without having advanced beyond the condition 

 noted on the previous day. It is possible for a few individuals 

 of this cross to live practically through the period of gastrula- 

 tion, but they are unable to enter upon embryonic differentia- 

 tion. It is to be noted, however, that this cross succeeds much 

 better than the reciprocal cross described in table 5 



TABLE 7 



Hybrids between F. heteroclitus 9 X Cyprinodon cf 



2 hours I About 20 per cent of eggs cleaving. Polyspermy evident in nearly 

 half of these eggs if one is to judge by the irregularity of blasto- 

 meres. The commonest rregularity consists of the production 

 of two cells of very unequal size 

 5 hours Nearly all that had cleaved are in advanced cleavage stages and 

 appear to be quite normal. Evidently early irregularities have 

 had little effect on the cell arrangements of this period 



12 hours Many eggs in advanced cleavage stages disintegrating. These are 

 doubtless the polyspermia eggs. Others still normal in appear- 

 ance 



20 hours All embryos dead, some evidently have but recently ceased to 

 develop. These had succeeded in completing cleavage but show 

 no signs of having entered upon gastrulation 



