176 Experimental Zoology 
The reciprocal cross, Strongylocentrotus ° and Spherechi- 
nus 2 gave less striking results, because of the greater difficulty 
in making the cross. In April, May, June, young stages were 
obtained that died. In July and August 29 per cent reached 
the pluteus stage. The hybrids showed no indication of their 
double origin, but were pure Strongylocentrotus (maternal). 
In November and December no eggs cross-fertilized. 
Doncaster has carried out experiments in hybridizing sea 
urchins that lead him to conclude that the different hybrids ob- 
tained by Vernon at different times of year owe their peculiarities 
to the temperature of the water in which they develop. Herbst 
has recently carried out a more elaborate series of experiments 
that lead him to a similar conclusion, although he thinks that 
some other condition than temperature is also operative. What 
the other condition is he did not determine; but he does not 
think that it can be due to the relative condition of ripeness of 
the male sex cells. In fact, his analysis of Vernon’s results, in 
the light of his own observations, seems to show, for the sperm 
at least, that Vernon’s evidence is most unsatisfactory. 
The preceding results apply more especially to the later larve 
or pluteus stages. In some respects the results seem to be in- 
consistent with results that other observers have obtained with 
the younger stages of these hybrids. Driesch has found, for 
instance, that the method of cleavage, its tempo, the character 
of the mesenchyme formation, and of gastrulation are char- 
acteristic of the egg irrespective of the kind of sperm that is used. 
In later stages, when the skeleton develops and the pigment ap- 
pears, the larvee first begin to show their hybrid origin. On the 
other hand, Boveri thinks that the hybrid characters appear 
very early, but the principal difference between his view and that 
of Driesch lies in the age at which each supposes the differences 
to become apparent. There can be little doubt, however, that, 
as a rule, in the early stages little or no trace of the paternal ele- 
ments appear, and only later do they influence the characters of 
the hybrid. This difference may be interpreted to mean that, 
at first, the elements introduced by the sperm — the nucleus or 
