FERTILISATION 



181 



and, according to Boveri, 1 induces the formation of a centrosome, 

 which, after the completion of fertilisation, initiates the process of 

 cell division. Cytoplasmic filaments arrange themselves around the 

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

 the male pronucleus, and afterwards comes to lie alongside of the 

 segmentation nucleus (as the nucleus- formed by the union of the 

 two pronuclei is called). In the segmentation nucleus the normal 



FIG. 56. Successive stages in the fertilisation of an ovum of Echinus 

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



number of chromosomes characteristic of the species is once more 

 restored. The oosperm, or zygote, produced in this way is the 

 starting-point of a long series of cell divisions which culminate in 

 the formation of a new, completely developed individual. 



1 Boveri, Zelleti Studien IV., Ueber die Natur der Centrosomen, Jena, 1901. 

 Jenkinson, "Observations on the Maturation and Fertilisation of the Egg of 

 the Axolotl," Quar. Jour. Micr. Science, vol. xlviii., 1904, has recently 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.) 



