188 THE PHYSIOLOGY OF REPRODUCTION 



centre of the cell, where it unites with the male pronucleus 

 which generally becomes somewhat enlarged. The middle- 

 piece of the spermatozoon also enters the egg, and, according to 

 Boveri, 1 induces the formation of a centrosome, which, after the 

 completion of fertilisation, initiates the process of cell division. 



i 



.-. I't.-"!.' - >..-' 



FIG. 50. Successive stages in the fertilisation of an ovum of Echinus 

 esculentus, showing the entrance of the spermatozoon. (From Bryce.) 



Cytoplasmic filaments arrange themselves around the centro- 

 some in the form of a star, the sperm-aster, which accompanies 

 the male pronucleus, and afterwards comes to lie alongside of 



1 Boveri, Zellen Studien IV., Ueber die Natur der Centrosomen, Jena, 

 1901. Jenkinson, " Observations on the Maturation and Fertilisation of the 

 Egg of the Axolotl," Qaar. Jour. Micr. Science, vol. xlviii., 1904, has re- 

 cently stated that the middle-piece of the spermatozoon, after forming the 

 centre of the sperm -sphere and sperm-aster, completely disappears, and that 

 the centrosome is formed from the sperm-nucleus at a later stage. (The 

 sperm-sphere is the clear area which forms in the ovum round the head and 

 middle-piece of the spermatozoon shortly after its entrance.) 



