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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. — Di.-\gram of the development of spermatozoa (upper line), of the 

 maturation and fertilisation of the ovum (lower line). 



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

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

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

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

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

 male nucleus (sp.ii) 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 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 



