314 THE CELL 



This, however, does not invariably occur. In cross-fertilisations, 

 which were made between Strongylocentrotus lividus and Sphaer- 

 echinus granular is, it was found, that out of the hundreds of ova, 

 which were experimented upon at various times, a varying num- 

 ber of eggs was produced, which had been fertilised by the strange 

 spermatozoa, whilst the large majority of ova were unaffected. 

 Thus we see, that the ova of the same animal differ from one 

 another, just as swarm-spores of the same species may react differ- 

 ently to light, some seeking the positive edge of the drop, others 

 the negative, and others, again, oscillating between the two (vide 

 p. 101). As swarm-spores exhibit different light reactions, the ova 

 of the same animal present different sex reactions, and what 

 is still more extraordinary, these sex reactions can be largely 

 influenced and altered by external circumstances. 



The experiment is a very simple one. The mature ova of 

 Echinoderms, after their evacuation from the ovaries, can be pre- 

 served in sea water in an unfertilised condition for 24-48 hours 

 without losing their capacity for development. But, during this 

 time, changes take place in them, which manifest themselves in 

 their behaviour towards foreign spermatozoa. 



Two different methods were adopted in the experiments, one of 

 which may be described as the method of successive after-fer- 

 tilisations. It consisted in this, that the experimenters crossed 

 the same egg-mass several times with foreign spermatozoa. In 

 doing this the following important result was obtained: all the 

 ova, which were crossed immediately after their evacuation from 

 the distended and full ovary, with extremely few exceptions, 

 refused the foreign spermatozoa; but after 10, 20, or 30 hours, 

 that is to say, after the second, third, or fourth crossing, an 

 increasingly large proportion of the ova behaved differently, 

 becoming cross-fertilised, and subsequently developing normally. 

 The same result was always produced, whether the ova of 

 Strongylocentrotus lividus were covered with the spermatozoa of 

 Sphasrechinus granularis, or of Echinus micro-tuber culatus, or 

 whether the ova of Sph&rechinus granularis were crossed with 

 the spermatozoa of Strong ylocenlrotus lividus. 



The success or failure of hybridisation cannot in these cases be 



attributed to a difference in the spermatozoa, since they were each 



time taken afresh from a distended and full testis, and may, 



herefore, be considered to be a relatively constant factor in the 



experiments. In this case, without doubt, it was the egg- cell 



