172 THE PHYSIOLOGY OF REPRODUCTION 



spermatocyte, or mother-cell, divides. (3) A product of division 

 of a spermatocyte divides, giving rise to a spermatid, the number 

 of chromosomes becoming reduced during this process to one- 

 half the previous number. Subsequently 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 con- 

 tinually going on in the seminiferous tubules of the testis, suc- 

 cessive crops of spermatozoa being from time to time produced. 

 The various stages of development may generally be observed 

 in the same testis, or even within the limits of a single tubule. 



It is supposed that the reduction in the number of the 

 chromosomes 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. 131). l 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. 



STRUCTURE OF SPERMATOZOA 



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 sper- 

 matozoon bores its way into the ovum. The tail of the sperm 

 consists of an 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. 

 Ballowitz 2 has shown that the axial filament is composed of a 

 number of parallel fibrilla?, like a muscular fibre. 



1 For an account of the process of spermatogenesis in different animals 

 and plants, and a discussion of the phenomena described, see Wilson, 

 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. 



2 Ballowitz, " Untersuchungen liber die Struktur der Spermatozoon," 

 Arch. f. Mikr. Anat., vol. xxxii., 1888, and vol. xxxvi., 1890; Zeitschr. f. wiss 

 Zool, vol. lx., 1890, and vol. Hi., 1891. 



