12 INTRODUCTION. 



Fertilisation, or impregnation, consists in the fusion of tlie 

 male element, or spermatozoon, wifcH the female element, or egg ; 

 or, more strictly speaking, fusion of the nuclei of these two bodies. 

 The spermatozoa of the male animal present certain points 

 of resemblance with the ova or eggs of the female. The early 

 stages of development of the two are closely similar, or even 

 identical, and at the time of first appearance of the reproductive 

 organs in the embryo it is impossible to say of which sex it will 

 subsequently become. In the developing testis, as in the ovary, 

 the essential elements are spherical cells, distinguished from 

 their neighbours by their larger size, and usually spoken of as 

 primitive ova, but better termed primary reproductive cells or 

 gonoblasts. In the female these gonoblasts become the per- 

 manent ova or eggs, usually directly, but sometihies after fusion 

 with one another. In the male each gonoblast, by repeated 

 division, gives rise to a number of cells which, by elongation, 

 become converted into the spermatozoa. In this formation of 

 spermatozoa a larger or smaller part of the gonoblast may take 

 no share, but remain unaltered as the blastophore, a portion 

 of granular protoplasm, round which the spermatozoa are 

 arranged. 



The fully formed spermatozoon is a cell, consisting of a head, 

 often rod-like in shape and composed almost entirely of the 

 nucleus, and a long vibratile tail by which the active move- 

 ments of the spermatozoon are effected. In being a single 

 nucleated cell, the spermatozoon resembles the egg or female 

 cell, as it does also in being derived from a primary reproduc- 

 tive cell or gonoblast. 



In other respects the male and female elements, spermatozoa 

 and ova, differ from each other markedly. The ovum is a large, 

 more or less spherical cell, with little or no power of movement. 

 The spermatozoon is a minute cell, usually of a rod-like shape, 

 and exhibiting active movements ; the spermatozoa of some 

 animals, as the Crustacea, are, however, spherical and motionless. 

 An ovum is very commonly formed by direct conversion from a 

 single primary reproductive cell or gonoblast, while in some 

 cases two or more gonoblasts may fuse to form a single ovum : 

 in the formation of the male elements, on the other hand, a single 

 gonoblast gives rise, not to one spermatozoon, but to a large 

 number ; all the spermatozoa derived from a single gonoblast 



