Hybrid and Parent Forms of Echinoid Larvce. 229 



much more readily than when freshly shed. Born, on .the other hand, 

 found the cross fertilisation could be increased by increasing the 

 amount of sperm added. Both methods, but especially the former, 

 were found of value in the present research. 



As regards the structure of the hybrids under discussion, it was 

 found that the majority of those obtained in May, June, and July 

 were of an almost pure Sphcerechinus type, only a third or less of 

 them being of an intermediate or Strongylocentrotus type. In 

 November, on the other hand, only about a sixth were of the maternal, 

 and five-sixths of the paternal type. Finally, in December and 

 January, all the hybrid larvae were of the paternal type. These 

 latter larvae in almost all cases showed obvious traces of their hybrid 

 origin, but they were evidently much more inclined to the Strongy- 

 locentrotus than to the Sphcerechinus type. Thus the body skeleton 

 was like that of Strongylocentrotus, only it generally had a few 

 abnormal projections springing from it. It was also, as a rale, about 

 25 per cent, shorter. The anal arm skeleton generally consisted of 

 two rods, but there were very seldom any cross bars joining them, 

 such as occur in Sphcerechinus larvae. 



On the reciprocal cross of Strongylocentrotus ? and Sphcerechinus $ 

 eighteen experiments were made. During April, May, and June a 

 fair number of the ova were cross-fertilised, but no plutei were 

 obtained. In July and August some 47 per cent, of the ova were 

 fertilised, and 29 per cent, of them survived to the eight clays pluteus 

 stage. In November and December, on the other hand, with one 

 exception, not only were no plutei obtained, but as a rule not a 

 single ovum was cross-fertilised. The hybrid larvas themselves were 

 of the pure Strongylocentrotus type, but in one instance the arms of 

 the larvae were very much longer than had ever been noticed in any 

 other case. These hybrids appeared therefore to be of the nature of 

 sport. 



These extraordinary variations in the capacity for cross-fertilisa- 

 tion seem to be due to the variations in maturity which the sexual 

 products undergo with change of season. Thus in the summer 

 months most of the Strongylocentrotus individuals contain but very 

 small quantities of ripe sexual products, or none at all. Also from 

 the fifty different series of observations made on normal Strongy- 

 locentrotus larvae, it appeared that the size of the larvae kept at the 

 maximum from the beginning of April till the beginning of May, 

 but then began to dwindle down, so that at the beginning of July it 

 reached its lowest level. The larvae were now sometimes 30 per cent, 

 smaller than the spring larvae. After the middle of August they 

 gradually increased again, and by the end of November had attained 

 their maximum size. Sphcerechinus larvae, on the other hand, kept at 

 about the same size throughout the year, though considerably 



