308 THE EVOLUTION THEORY 



that from sea-urchin sperms which have been killed by heat it is 

 possible to extract in aqueous solution a substance capable of exciting 

 imfertilized sea-urchin eggs to development, although they only go as 

 far as to the sixteen-cell stage. 



From all these results we can at least infer so much, that 

 chemical changes and influences may determine whether the ripe 

 ovum shall go on to embryonic development or not, and that these 

 influences may be very diverse in nature in different cases. I shall 

 return later to these important facts. 



When we sum up the facts we have cited with reference to the 

 reduction of the number of chromosomes, it appears that nature is, 

 as it were, striving to keep the number constant for each species; 

 that in germ-cells which are destined for amphimixis they are reduced 

 to half the normal number, but that this halving of the number is 

 suppressed where fertilization is always absent, or that the reduction 

 to half is compensated for again in various ways, whether by sub- 

 sequent fusion of the two daughter-nuclei, which have arisen from 

 the process of reduction, or by an independent duplicating of the 

 chromosomes in the segmentation nucleus. 



We might perhaps be inclined to conclude from all this that the 

 occurrence of development depended on the presence of the normal 

 number of chromosomes ; and I used to regard this as possible. But 

 facts which have been more recently brought to light have excluded 

 this view. Above all, we now know that every nuclear division 

 depends on the presence of a dividing apparatus, a centrosphere, but 

 that this organ degenerates in the ova of most animals and is com- 

 pletely lost after the second polar division has been effected. The 

 mature ovum is therefore in itself incapable of entering on its 

 embryonic development, no matter how many chromosomes its 

 nucleus contains; it is only capable of further division when the 

 fertilizing sperm-cell brings with it its dividing apparatus or centro- 

 sphere. In thread-like sperms this lies in the median portion (Fig. 

 68 C), and after the tail-piece has been dissolved, which happens soon 

 after the sperm enters the egg, the central corpuscle, at first very 

 small, can be recognized in front of the sperm-nucleus, where it is 

 scon transformed into an ' aster ' and divides into two. Then both 

 sphei-es move apart (Fig. 75 B, p. 396) and form the nuclear spindle 

 between them by the confluence of their rays. 



From this the division of the ovum into the two first embryonic 

 cells proceeds. The two pronuclei in the ovum, the male and the 

 female, are thus exactly alike as to number of the chromosomes, and 

 frequently at least as to. size and appearance (Fig. 75 C). But they 



