FERTILISATION 



189 



the segmentation nucleus (as the nucleus formed by the union 

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

 the normal 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. 



Jenkinson, who has carried out a series of experiments in- 

 tended to elucidate the physical processes occurring in fertihsa- 

 tion, draws the conclusion that the structures which appear in 

 the ovum are produced under the influence of the middle-piece 



Fig. 51. — Three stages in the conjugation of male and female nucleus 

 in the fertilisation process of Echinus, (From Brjce.) 



and centrosome. He supposes these bodies to possess the power 

 of withdrawing water from the cytoplasm, of swelling up and 

 dissolving in the water so absorbed, and then giving off radial 

 outgrowths which precipitate the proteins of the cell, and so 

 produce the fertihsation spindle. Jenkinson lays some stress 

 on the fact that a watery fluid collects in vacuoles in the centre 

 of the sperm-sphere of the axolotl, and regards the presence of 

 this fluid as evidence that the sperm introduces a hydroscopic 

 substance into the ovum. In confirmation of this the experi- 

 ments show that a hydroscopic particle is capable of giving rise 

 to an astral structure in a coUoid solution. ^ 



Boveri and others have proved experimentally that portions 

 of imfertihsed Echinoderm ova, without egg nuclei, may develop 

 ' Jenkinson, loo. cit. Further references are given in this paper. 



