REPRODUCTION, 61 
latter is not essential, spermatozoa without such an appendage 
do occur. The obvious purpose of the cilium is to convey the 
male cell to the ovum through a fluid medium—either the water 
in which the ova are discharged in the case of most inverte- 
brates, or through the fluids that overspread the surfaces of the 
female generative organs. 
The Origin of the Spermatozodn.—The structures devoted to 
the production of male cells (testes), when reduced to their es- 
Fie. 65.—Spermatogenesis. A—H, isolated sperm-cells of the rat, showing the development 
of the sper mataeatn and the gradual transformation of the nucleus into the spermatozoén 
head. In G the seminal granule is being cast off (after H. H. Brown). I—M, sperm-cells 
of an Elasmobranch. The nucleus of each cell divides into a large number of daughter- 
nuclei, each one of which is converted into the rod-like head of a spermatozoén. _N, trans- 
verse section of a ripe cell, showing the bundle of spermatozoa and the passive nucleus 
(I—N, after Semper). O—S, spermatogenesis in the earth-worm: O, young sperm-cell ; 
P, the same divided into four; Q, spermatosphere with the central sperm-blastophore ; 
R, a later stage ; S, nearly mature spermatozoa. (After Blomfield.) 
sentials, consist of tubules, of great length in mammals, lined 
with nucleated epithelial cells, from which, by a series of 
