AETiriCIAL PARTHENOGENESIS IN ECHINUS ESCULENTUS. 531 
While it is fi’equently possible to get good ripe females well 
into June, most of the females have shed their eggs by the 
iniddle of May. 
The exact time when the breeding period may be said to 
reach its tnaximum, seems to vary very considerably, some 
years being much nearer the former than the latter part of the 
season. This variation is influenced by temperature and other 
■conditions. Of the two years throug-h which this work has 
been extended the first may be said to have been normal in 
that the breeding season reached its height at the usual time, 
in the middle of April. The fertilisation controls went well, 
and the growth of the larvae was rapid and vigoi’ous. 
In the season of 1912, however, good ripe females were 
much harder to obtain, and often, although the eggs of some 
of these looked quite ripe, they failed to segment when 
fertilised witli sperm. Moreover, in our hybridisation experi- 
ments of 1912 we found the same trouble. We have some 
evidence, then, for supposing that the second season was not 
so favourable as the first for our experiments. 
In 1911 we had no difficulty in getting a large number of 
plutei to reach a late period, and quite a nutnber to pass 
through metamorphosis to the young urchin stage. In the 
season of 1912 very few reached metamorphosis, and none of 
them advanced as far as in the previous year. This difference 
in the two years should be kept in mind in comparing the 
results of the two years. 
It is possible that the combined method of Loeb and 
Delage that 'we adopted during the second season has not so 
far, for these reasons, had a fair trial. The fact that we did 
not obtain young inchins by this method, may have its 
explanation in our possibly using unfavourable material for 
the majority of our experiments. It should also be remem- 
bered that we have been equally unsuccessful this year with 
Loeb’s method. Whatever the differences between the two 
seasons may have been, it affected only the later stages of 
development, for little difference could be noticed between the 
numbers of blastulae obtained, during the early stages of the 
