xv] NUCLEUS IN FOREIGN CYTOPLASM 243 



high degree of incompatibility between the egg-cytoplasm 

 and the sperm-nucleus (e.g. Echinoid eggs and Mollusc 

 sperm), the sperm-nucleus is unable to resolve itself into 

 chromosomes, or, if it does so, that these take no part in 

 mitosis. If the incompatibility be rather less, the chromo- 

 somes may be formed to all appearance normally, and 

 divide in the usual way on the spindle, but nevertheless they 

 may be so alien to the cytoplasm in which they exist, that 

 they exert no influence upon it, and the characters of the 

 resulting larva are as they would be if the egg developed 

 parthenogenetically, that is to say, purely maternal. 



Finally, if the incompatibility is relatively slight, the 

 chromosomes may be able to adapt themselves to their 

 strange surroundings, and not only continue to live and 

 divide, but also to have some effect on the characters of the 

 larvae or adults produced, as happens in generic crosses 

 between Sea-urchins. It is impossible at present either to 

 prove or to disprove this suggestion, but until it is disproved 

 such results as those obtained by GODLEWSKI cannot be used 

 as a final argument against the chromosome hypothesis, at 

 least with regard to characters that appear later than the 

 very early larval stages. The observation that gastrulae 

 from non-nucleated Echinoid egg-fragments fertilised by 

 Antedon sperm are of the Echinoid rather than of the Cri- 

 noid form does of course disprove that all characters, at 

 every stage of development, are determined by factors 

 borne by chromosomes, but as will be seen immediately, 

 there is ample experimental evidence to show that the very 

 early stages of development are controlled largely, if not 

 completely, by the egg cytoplasm. 



Although, therefore, several of the experiments that are 

 often quoted as showing that the cytoplasm is no less im- 

 portant than the chromosomes in transmitting inherited 



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