226 THE PHYSIOLOGY OF REPRODUCTION 



consequent upon gametic union are the result of a catalytic 

 chemical reaction is in no way opposed to the vaguer physio- 

 logical conception that the object of the process is to secure 

 a rejuvenescence of vital substance without which the race 

 cannot be perpetuated. 



The cytological changes which occur in artificially fertiUsed 

 ova have been dealt with at considerable length by Wilson, to 

 whose paper ^ the reader is referred. It is shown that the ovum 

 of the sea-urchin, under an appropriate stimulus, is able to 

 construct the complete mechanism of mitotic cell division 

 without the importation of a sperm-centrosome, but beyond 

 this a multitude of aberrations are exhibited. The number of 

 chromosomes is one-half that occurring in normally fertihsed eggs, 

 being in the sea-urchin eighteen instead of thirty-six. The cen- 

 trosomes are primarily formed de novo. According to Delage,^ 

 however, the number of chromosomes in artificially fertihsed 

 sea-urchins becomes eventually restored to the normal by a 

 process of auto-regulation. 



1 Wilson, " Experimental Studies in Cytology : I. A Cytological Study of 

 Artificial Parthenogenesis in Sea- Urchin Eggs," Arch. f. Entwick.-Meehanik, 

 vol. xii., 1901. For an account of the cytological phenomena in normal 

 parthenogenetic eggs, especially in insects, see Hewitt, " Cytological Aspects 

 of Parthenogenesis in Insects," Memoirs and Proe. Manchester LiUrary and 

 Philosoph<coi,l Soc, vol. 1., 1906. 



2 Delage, "Etudes experimentales snr la Maturation Cytoplasmique chez 

 les Echiuodermes," Anh. de Zool. Exper. et. Qin.,vo\. ix. , 1901. Of. also 

 Tennent and Hogue, " Studies on the Development of the Starfish Egg," 

 Jour, of Exp. Zool., vol. iii., 1906. 



