194 



Tlie Study of Animal Life part in 



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). 



primitive amoeboid se.v-cell ; A, ovum with nucleus ('/); B, ovum e.xtriiduig 

 the first polar body (/I) and leaving the nucleus («^) reduced by half ; C, 

 extrusion of the second polar body (/"), the nucleus («-) now reduced to a 

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

 spermatozoa (.v/) ; D, the entrance of a spermatozoon into the ovum ; E, the 

 male nucleus {sp.n) and the female nucleus («-) approach one another, and 

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

 Evolution 0/ 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 beg^ins 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 



