STUDIES ON THE DEVELOPMENT OP ECHINOIDEA. 317 
esculent us is very similar to that undergone by these eggs 
when fertilised with the sperm of their own species. Fig. 12 
shows the appearance of the free-swimming blastula twenty- 
four hours fertilisation. In this specimen the vegetative end is 
rather broader than the animal end, whereas in the blastula of 
Echinocurdium represented in fig. 7, the reverse is the case, 
but this difference between the hybrid and normal blastula 
is not constant. By the end of the second day the blastula 
is converted into a gastrula such as is shown in fig. 13. The 
rate of development of the hybrid, however, varies with the 
specimens used as parents. In fig. 14 a hybrid larva of 
the same age is represented, which, however, belonged to a 
different culture, and we can see that it has attained the stage 
where the calcareous stars have been formed and where the 
coelom is already grooved off from the archenteron. If from 
the same specimen eggs are taken some of which are fertilised 
with sperm of its own species and some with the sperm of 
Echinus esculentus, then the hybrids will always develo]) 
more slowly than the normal larvge. As development pro- 
ceeds the hybrids fall Jiiore and more behind the normal 
larvse. In fig. 15 a hybrid four days old is represented. In 
this specimen the post-oral arms are well developed but the 
stomodasum has not joined the oesophagus. The bars sup- 
porting these arms are “latticed,” but the cross bars are 
comparatively few. The antero-lateral bars ai-e there but the 
autero-lateral arms are not yet developed. As shown in fig. 
16, however, hybrids four days old may be more advanced in 
development. In the s])ecimen shown in this figure the 
alimentary canal is complete and the ])ost-oral arms are longer 
than in the specimen shown in fig. 15. But the supporting- 
bars of these arms are single rods for the greater 
])art of their length, although a second short rod accom- 
panies the first at its base, as it does as a variation in 
Echinus esculentus. Both larvm agree in the total 
absence of any indication of the aboral spike or of 
its skeleton. 
When we turn our attention to the hybrid larvm five days 
