176 Experimental Zoology 



The reciprocal cross, Strongylocentrotus 9 and Sphaerechi- 

 nus $ 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 Hght 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 larvae 

 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 larvae 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 Httle 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 



