.230 FERTILIZATION OF THE OVUM 



cell loses most of its cytoplasm, the main bulk of which, and hence 

 the main body of the embryo, is now supplied by the egg ; and 

 in the higher plants, the egg alone retains the plastids which 

 are thus supplied by the mother alone. On the other hand, the 

 paternal germ-cell is the carrier of something which incites the egg 

 to development, and thus constitutes the fertilizing element in the 

 narrower sense. There is strong ground for the conclusion that in 

 the animal spermatozoon this element is, if not an actual centro- 

 some, a body or a substance directly derived from a centrosome of 

 the parent body and contained in the middle-piece. Boveri's theory, 

 according to which fertilization consists essentially of the replace- 

 ment of a missing or degenerating egg-centrosome by the importation 

 of a sperm-centrosome, was stated in too simple and mechanical a 

 form ; for the facts of spermatogenesis show conclusively that the 

 spermatid-centrosome is not simply handed on unmodified by the 

 spermatozoon to the egg, and the theory wholly breaks down in 

 the case of the higher plants. But although the theory probably 

 cannot be sustained in its morphological form, it may still contain 

 a large element of truth when recast in physiological terms. Like 

 mitosis, fertilization is perhaps at bottom a chemical process, the 

 stimulus to development being given by a specific chemical substance 

 carried in some cases by an individualized centrosome or one of its 

 morphological products, in other cases by less definitely formed 

 material. In the case of animals, we cannot ignore the historical 

 continuity shown in the origin of the spermatid-centrosomes, the 

 formation of the middle-piece, and the origin of the sperm-centro- 

 somes and sperm-amphiaster in the egg, even though we do not 

 yet know whether the sperm-centrosome is as such imported into 

 the egg. And this chain of phenomena suggests that even in the 

 higher plants, where no centrosomes seem to occur, the fertilizing 

 substance, even if brought into the egg in an unformed state, may 

 still be genetically related to the mitotic apparatus of the preceding 

 division. 1 



Through the differentiation between the paternal and germ-cells 

 in the higher forms indicated above, their original morphological 

 equivalence is lost and only the nuclei remain of exactly the same 

 value. This is shown by their history in fertilization, each giving 

 rise to the same number of chromosomes exactly similar in form, 

 size, and staining-reactions, equally distributed by cleavage to the 

 daughter-cells, and probably to all the cells of the body. We thus 

 find the essential fact of fertilization and sexual reproduction to be a 

 union of equivalent nuclei ; and to this all other processes are tributary. 



As regards the most highly differentiated type of fertilization and 



1 Cf. Strasburger's view, p. 221. 



