360 
The development during the cleavage stages, similar to the reciprocal 
eross, 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, nearing 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. 
Fundulus heteroclitus, female, 
x 
Menidia 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 impreguated was as follows: 
Ezperiment 119 
5 501... 
PON iste aw ret ke are Axe ay .....81 per cent. 
In experiment 501 the wet method was employed whick probably is re- 
sponsible fer the low percentage. The controls with normal Fundulus eggs 
showed a correspondingly low per cent. of impregnation. 
The number of dispermic and pelyspermic eggs was considerably less 
than in the cross with Menidia notata. In experiment 503 the per cent. was 
thirteen, about two-thirds of which were disperimic. 
The rate of development and the stage at which it stops is similar to 
that of the cross with Menidia notata. As a whole the number of eggs that 
suecessfully effect the closure of the blastopore is greater and the embryos 
yary considerably less in their lengths, approaching more nearly to the nor- 
