112 BLASTOGENIC VAKIATIONS. 



the whole of the hybrid larvae from the Sphcerechinus 

 to the Strongylocentrotus type. A repetition of these 

 crossing experiments in a subsequent year * confirmed 

 the conclusion that the summer hybrids were more in- 

 clined to the Sphcerechinus type than the winter ones, 

 though on this occasion they were only very rarely 

 found to approach to the pure Splicer echinus type. 



The reciprocal cross of Strongylocentrotus ova with 

 Splicer echinus sperm illustrates still another way in 

 which the sex-cells may be affected by changes in ma- 

 turity and nutrition. Thus during April, May, and 

 June a fair number of the ova were cross-fertilised, 

 though no plutei were obtained: but in July and August 

 some 47 per cent, of the ova were fertilised, and 29 per 

 cent, of them survived to the eight days 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. In other 

 words, the Strongylocentrotus $ Sphcerechinus $ hy- 

 brid is only formed at a time when the Strongylocen- 

 trotus ova have reached their minimum maturity. 



The observations made upon these sea-urchin larvse 

 thus afford conclusive evidence that changes in the con- 

 ditions of nutrition of the sex-cells produced by keeping 

 them in sea water may affect the size of the larvae both 

 in a positive and negative direction, whilst changes of 

 nutrition dependent on season may produce a much 

 more considerable effect on size, and may in some in- 

 stances so alter the nature of the germ-plasm as to give 

 rise to most marked variations of structure in the re- 

 sulting hybrid offspring, and in other instances largely 

 * Arch, f . Entwickelungsmechanik, Bd. ix. p. 464, 1900. 



