FERTILIZATIOX IX PLAXTS 21 5 



centrosome be no longer visible. On such a basis we may perhaps 

 find a reconciliation between these observations and Boveri's theory, 

 and may even bring the fertilization of plants into relation with it 

 (p. 22 1). Even in case of the nucleus, universally recognized as a 

 permanent cell-organ, it is not the whole structure that persists as 

 such during division, but only the chromatin-substance — in some 

 cases only a small fraction of that substance. The law of genetic 

 continuity therefore would not fail in case of the centrosome, though 

 only a portion of its substance were handed on by division ; and even 

 if we take the most extreme negative position, assuming that the 

 sperm-centrosome is wholly formed anew under the stimulus of the 

 spermatozoon, we should still not escape the causal nexus between 

 it and the centrosome of the spermatid. 



Boveri himself has suggested ^ that the egg may be incited to 

 development by a specific chemical substance carried by the sperma- 

 tozoon, and the same view has been more recently urged by Mead,^ 

 while Loeb's recent remarkable experiments on sea-urchins ('99) show 

 that the e.gg may in this case {Ardacm) undergo complete parthe- 

 nogenetic development as the result of artificial chemical stimulus.^ 

 Assuming such a substance to exist, by what part of the spermato- 

 zoon is it carried ? It is possible that the vehicle may be the nucleus, 

 which forms the main bulk of that which enters the egg ; and this 

 view seems to be supported by what is at present known of fertiliza- 

 tion in the plants (p. 221). Yet when we regard the facts of fertili- 

 zation in animals, taken in connection with the mode of formation of 

 the spermatozoon, we find it difficult to avoid the conclusion that the 

 substance by which the stimulus to development is normally given is 

 originally derived from the spermatid-centrosome, is conveyed into 

 the egg by the middle-piece, and is localized in the sperm-centro- 

 somes which are conveyed to the nuclear poles during the amphi- 

 aster-formation. Accepting such a view, we could gain an intelligible 

 view of the genetic relation between spermatid-centrosome, middle- 

 piece, sperm-centrosome, and cleavage-centrosomes, without commit- 

 ting ourselves to the morphological hypothesis of the persistence of 

 the centrosome as an individualized cell-organ. Such a conclusion, 

 I believe, would retain the substance of Boveri's theory while leaving 

 room for the abandonment of the too simple morphological form in 

 which it was originally cast. 



D. Fertilization in Plants 



The investigation of fertilization in the plants has always lagged 

 somewhat behind that of the animals, and even at the present time 



1 '91, p. 431, 2 '98, 2, p. 217. ^C/.p.iii. 



