194 



The Study of Animal Life PART m 



element belonging to the pollen cell unites with the nucleus 

 of the egg-cell. The union is intimate and complete. 



When spermatozoa come in contact with the egg-shell 

 of a cockroach ovum, they move round and round it in 

 varying orbits until one finds entrance through a minute 

 aperture in the shell. It works its way inwards until its 

 nuclear part unites with that of the ovum. The union is 

 again intimate and complete. 



FIG. 36. Diagram of the development of spermatozoa (upper line), of the 



maturation and fertilisation of the ovum (lower line). 



a, primitive amoeboid sex-cell; A, ovum with nucleus (); B, ovum extruding 

 the first polar body (/!) and leaving the nucleus (n\) reduced by half ; C, 

 extrusion of the second polar body (/ 2 ), the nucleus ( 2 ) now reduced to a 

 fourth of its original size ; i, a mother-sperm-cell, dividing (2 and 3) into 

 spermatozoa (sp) ; D, the entrance of a spermatozoon into the ovum ; E, the 

 male nucleus (sp.n) and the female nucleus (n 2 ) approach one another, and 

 are about to be united, thus consummating the fertilisation. (From the 

 Evolution of Sex.} 



Both in plants and in animals the male cell is attracted 

 to the female cell, the two nuclei unite thoroughly, and, 

 when fertilisation is thus effected, the egg-cell is usually 

 impervious to other sperms. 



A single nucleus of double origin is thus established, 

 and the egg-cell begins to divide. Some idea both of the 

 orderly complexity of the nuclear union and of the careful- 

 ness of modern investigation may be gained from the fact 

 that the nuclei of the two daughter-cells which result from 



