360 



The development during the cleavage stages, similar to the reciprocal 

 cross, proceeds normally. It is only in the subsequent stages that the 

 effect of hybridization manifests itself. This shows itself for one thing in 

 the great irregularity of the stages at a given moment. At a time when 

 some of the eggs have proceeded as far as they will go, the greater number 

 of the eggs are in all stages, back to the close of cleavage. This is much 

 more marked than in the reciprocals. It is possible, however, that this 

 is a function of the egg, since even the normals show a considerably greater 

 number of stragglers than do the normal Fundulus eggs. The eggs of 

 this species are evidently less hardy and thus may lend themselves less 

 perfectly to the methods used in rearing them. When development finally 

 ceases the embryos are, for the most part, uearing the closure of the blasto- 

 pore, the more successful ones showing an embryo with the optic vesicles, 

 but with the body shorter than the normals. The conditions are not essen- 

 tially different from that described for the reciprocals, except that, as a 

 whole, the development gives out at a somewhat earlier period. This, as 

 already indicated, is possibly due to the less hardy condition of the Menidia 

 egg. 



FidkUiJiis hetcrnrlitits. female, 



X 



Menklia gracilis, male. 



Menidia gracilis is distinguished with difficulty from Menidia notata, 

 except in its smaller size. Three experiments were made with this cross. 

 The percentage of eggs impregnated was as follows: 



Ezperiment 119 93 per cent. 



" 501 About 5 per cent. 



503 81 per cent. 



In experiment 501 the wet method was employed which probably is re- 

 sponsible for the low percentage. The controls with normal Fundulus eggs 

 showed a correspondingly low per cent, of impregnation. 



The number of dispermic and polyspennic eggs was considerably less 

 than in the cross with Menidia notata. In r'xperiment ."i((;l the per cent, was 

 thirteen, about tv/o-thirds of which ^\-er<' (li-^pcrniic. 



The rate of development and the stnue at which il stops is similar to 

 that of the cross Avith Menidia notata. As a whole the number of eggs that 

 successfully effect the closure of the blastopore is greater and the embryos 

 vary considerably less in their lengths, approaching more nearly to the nor- 



