i66 THE PHYSIOLOGY OF REPRODUCTION 



The changes which occur ia spermatogenesis may be summarised 

 as follows : (1) A spermatogonium divides into two, one product of 

 division passing into the second layer of tlie seminal epithelium 

 and becoming a spermatocyte. (2) A primary spermatocyte, or 

 mother-cell, divides, the number of chromosomes becoming reduced 

 during this process to one-half the previous number. (3) A secondary 

 spermatocyte so formed divides, giving rise to a spermatid. Sub- 

 sequently the spermatid elongates, the nucleus becomes shifted to 

 one end, and the spermatozoon which is formed in this way is set 

 free. The process is continually going on in the seminiferous tubules 

 of the testis, successive crops of spermatozoa being from time to 

 time produced. The various stages of development may generally 

 be observed in thb same testis, or even within the limits of a single 

 tubule.^ 



It is supposed that the reduction in the number of the chromo- 

 somes is a preparation of the conjugating cells for their subsequent 

 union in fertilisation,, and is a means by which this number is held 

 constant in the species (see p. 126).^ In those animals in which 

 reproduction is normally effected without the intervention of a 

 spermatozoon (parthenogenesis) the ovum may discharge only one 

 polar body instead of two. 



Stbuctuee of Speematozoa 



A fully developed human spermatozoon consists of a flattened 

 egg-shaped head, a short cylindrical body or middle-piece, and a long 

 delicate vibratile tail. Lying anterior to the head is a small apical 

 body, or achrosome, which in some animals bears a little barb-like 

 projection by means of which the spermatozoon bores its way into 

 the ovum. The tail of the sperm consists of ah axial filament 

 surrounded by a protoplasmic envelope, which becomes, very thin or 

 disappears altogether at the extremity, leaving a short naked end- 

 piece. The axial filament passes anteriorly through the middle-piece, 

 and ends in a small knob (the end-knob) at the base of the head. 



1 The process of reduction may result in two sorts of spermatozoa being 

 produced, these after fertilisation giving rise respectively to the two sexes, in 

 whose body cells the chromosomes may differ in number or in kind ; that is to 

 say, one sex may carry two chromosomes specially associated with sex and called 

 the " X chromosomes," while the other sex only carries one X chromosome, with 

 or without another smaller chromosome called the " Y chromosome " (see 

 Chapter XV. below). 



^ For an account of the process of spermatogenesis in different animals and 

 plants, and a discussion of the phenomena described, see Wilson (E. B.), The Cell 

 in Development and Inheritance, 2nd Edition, London, 1900. In this work the 

 theories of Weismann and others are dealt with, and a full account of the 

 literature is given up to 1900. For later work see Donoaster, An Introduction 

 to the Study of Cytology, Cambridge, 1920. 



