1903.] 



Experiments in Hybridisation, 



497 



£i Experiments in Hybridisation, with Special Keference to the 

 Effect of Conditions on Dominance." ByL. Doncaster, B.A., 

 King's College, Cambridge. Communicated by Dr. S. F 

 Harmer, F.K.S. Eeceived March 19,— Bead May 7, 1903. 



(Abstract.) 



The paper describes experiments made at Naples with hybrid 

 Echinoid larvae. The object of the experiments was to determine 

 whether the dominance of a character is influenced by the condition 

 of the genital cells at the time of fertilisation. It had been suggested 

 by Vernon* that the "prepotency" of the sexual cells varies with 

 their maturity, and experiments were made to test this conclusion, 

 and also to discover whether " prepotency " could be influenced by 

 other conditions acting on the eggs or spermatozoa before fertili- 

 sation. 



It was found that adverse conditions acting on the eggs did give 

 rise to differences in the larvae, but evidence is given to show that 

 these differences are not due to a change in the dominance of characters, 

 but are the result of differences in the vigour of the larvge. It is also 

 shown that the seasonal changes observed by Vernon are probably due 

 chiefly, if not entirely, to differences of temperature,: and are not 

 caused by a change of dominance accompanying difference of maturity. 



It is also shown that if an individual, A, shows greater dominance 

 than B when each is crossed with a specimen X of the other sex, then 

 A will also show greater dominance than B when both are crossed with 

 a specimen Y. 



It is shown that the different characters of one parent are inherited 

 separately by the hybrid offspring, so that there is no pronounced 

 correlation in the offspring between characters derived from the same 

 parent. Further, a given character may appear in very different 

 degrees on the two sides of the body of a hybrid larva, so that the 

 hybrids are very frequently asymmetrical, although in the characters 

 ■considered the two sides of the pure-bred larvae are similar. 



Experiments are described dealing with the causes which hinder 

 ■cross-fertilisation between separate species, and it is shown that treat- 

 ment of the eggs which tends to reduce their vitality usually renders 

 their fertilisation by sperm of another species more easy. 



* 'Phil. Trans.,' B, 1898. 



