THE VITAL PROPERTIES OF THE CELL 315 



alone that altered its behaviour towards the foreign sperma- 

 tozoa. 



Hence, if changes take place, or can be induced artificially to 

 take place, in the egg-cell, by means of which hybridisation is 

 rendered practicable, we must conclude, from a theoretical point 

 of view, that it is also possible to induce so complete a hybridisa- 

 tion between the sexual products of two species, which have a 

 certain degree of sexual affinity for one another, that scarcely any 

 ova should remain unfertilised. Thus, according to the conditions 

 under which the sexual products are brought together, a maximum 

 or a minimum of hybridisation may be obtained. 



In order to establish these relations, it is best, in making the 

 experiments, to divide the egg-material of a female into several 

 portions, which are fertilised at different times. The smallest per- 

 centage of hybrids is always obtained when the foreign spermatozoa 

 are added to the ova immediately after these latter have been 

 evacuated from the ovaries. The later fertilisation takes place, 

 whether after 5, 10, 20 or 30 hours, the greater is the percentage 

 of the hybridised ova, until the maximum of hybridisation is 

 reached. This is called the stage at which the addition of foreign 

 spermatozoa produces normally the greatest possible number of 

 eggs. This period is of short duration, since imperceptible 

 changes in the ova are uninterruptedly taking place. After that, 

 the percentage of the ova which, in consequence of the bastard 

 fertilisation, develop normally, begins to decrease ; and this is due 

 to the fact, that a steadily increasing number of ova are caused to 

 segment in an abnormal fashion and to become malformed, in 

 consequence of several spermatozoa having penetrated into each of 

 them. 



The results obtained by fertilising eggs at different times may 

 be represented by a curved line, the summit of which corresponds 

 to the maximum of hybridisation. The results obtained by cross- 

 ing the ova of Sphserechinus granularis with the spermatozoa of 

 Strongylocentrotus serve as an illustration. When fertilisation 

 takes place a quarter of an hour after the eggs have been evacu- 

 ated from the ovary (minimum hybridisation), only a very few 

 individual ova are developed. After two and a quarter hours 10 

 per cent, can be fertilised, after six and a quarter hours about 

 60 per cent., whilst after ten and a quarter hours almost all the 

 ova, with the exception of about 5 per cent., are affected ; in the 

 latter case they generally develop normally (maximum hybridisa- 



