530 CRESSWELL SHEARER AXD DOROTHY JORDAN LLOYD. 
aud as fai’ as tins is concerned^ have not found that they need 
any special food other than that used for feeding the normal 
fertilised larvae. Our young urchins have in most cases, as 
we have said, failed to develop their mouths sufficiently 
soon after metamorphosis to feed on the algte we have found 
so satisfactory in our Echinoderm hybridisation experiments. 
One remarkable fact which we have repeatedly noticed 
is that the eggs that develop and segment best, after 
treatment with various chemical solutions, are not those that 
are in the very best condition for fertilisation with sperm. 
In our experiments in Echinoderm hybridisation we have 
learnt to distinguish what are more or less the ripest eggs 
for giving the best results on fertilisation with sperm. This 
consists in the absence of nucleus and a peculiar appearance 
of the cytoplasm, which, although somewhat hard to describe, 
is readily learnt from practical observation. We have been 
surprised, therefore, that invariably those eggs, which we 
concluded from our observations were in the best condition 
€ 
for giving the highest percentage of normal fertilisation 
with sperm, were just those that gave poor results by methods 
of artificial parthenogenesis. We have repeatedly remarked 
that those lots of eggs in which our fertilisation controls 
with sperm gave very high percentages, gave very poor 
results by any method of artificial parthenogenesis we 
might use. In fact, early in our work we soon learnt to dis- 
tinguish the peculiar stage of ripeness necessary to give 
good results by artificial fertilisation, and this stage of ripe- 
ness is quite different from that which experience has taught 
us is most suitable for normal fertilisation with sperm. This 
has been so constant a feature in all our experiments that 
there is no doubt that it is a definite factor which should be 
taken into account in considering the nature of the chemical 
changes which accompany artificial parthenogenesis, at least 
under the conditions presented at Plymouth. 
The breeding’ period of E. esculentus is confined at 
Plymouth to a relatively short period, extending from the end 
•of February, in favourable years, to the first part of May. 
