182 THE PHYSIOLOGY OF REPRODUCTION 



Jenkinson, who has carried out a series of experiments intended 

 to elucidate the physical processes occurring in fertilisation, draws 

 the conclusion that the structures which appear in the ovum are 

 produced under the influence of the middle-piece 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 fertilisation 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 



FIG. 57. Three stages in the conjugation of male and female nucleus 

 in the fertilisation process of Echinus. (From Bryce.) 



experiments show that a hydroscopic particle is capable of giving 

 rise to an astral structure in a colloid solution. 1 



Boveri and others have proved experimentally that portions of 

 unfertilised Echinoderm ova, without egg nuclei, may develop 

 normally after the addition of spermatozoa, while Driesch has 

 shown that if such ova are deprived of their envelopes by shaking, 

 and are then divided into fragments some of which contain no 

 nuclei, the latter are capable of being fertilised a second time. It 

 is clear, therefore, that in such cases the union of nuclei is not 

 essential for the development of the ovum. 2 



In those ova which are surrounded by a membrane it is probable 

 that the fertilising spermatozoon bores its way through at any point 

 (Mammals and Amphibians). In other cases there is a small aperture 

 in the wall of the ovum ; this is called the micropyle (some Pisces 

 and Insecta). Some eggs, however, are naked, so that the sperms 



1 Jenkinson, loc. cit. Further references are given in this paper. 



2 For references to the original papers, which are somewhat numerous, see 

 Przibram, Embryogeny, English Translation, Cambridge, 1908. 



