872 REPRODUCTION, DEVELOPMENT, GROWTH AND DEATH [CH. LIX. 



coat is, however, very thick, and is made up of two strata imperfectly 

 separated by connective tissue and blood-vessels. Of these the 

 thinner outer division is the true muscular coat, the fibres of which 

 are arranged partly longitudinally, partly circularly. The inner 

 division is very thick, its fibres run chiefly in a circular direction ; 

 the extremities of the uterine glands extend into its internal surface. 

 It is in fact a much hypertrophied muscularis mucosse. The 

 mucous membrane is thick and consists of a corium of soft con- 

 nective tissue, lined with ciliated epithelium; this is continued down 

 into long tubular glands which have, as a rule, a convoluted course. 

 In the cervix the glands are shorter. Near the os uteri the epi- 

 thelium becomes stratified; stratified epithelium also lines the 

 vagina. 



THE PHYSIOLOGY OF THE REPRODUCTIVE ORGANS 



The object of the ovary is to produce ova ; this is known as 

 oogenesis. The object of the testis is to produce spermatozoa ; this is 

 known as spermatogenesis. The prodigality of nature in providing 

 for the continuance of the species is well illustrated by the fact that 

 at birth the human ovary contains about 70,000 immature oocytes. 

 Quite a small minority of these attain maturity, and get situated in 

 Graafian follicles: many Graafian follicles, moreover, never burst; 

 after attaining a certain degree of maturity, even during childhood, 

 they atrophy more or less completely. On the average, one Graafian 

 follicle ripens every four weeks, so that in the period between the 

 onset of puberty and the menopause, say from fifteen to forty-five 

 years of age, there is a possibility in the thirty intervening years of 

 the production of about 400 ripe ova. Of these again a very small 

 minority become fertilised. Still more is the lavishness of the 

 provision illustrated in spermatogenesis ; it has been calculated that 

 in the semen ejaculated at an act of coitus there are more than two 

 hundred million spermatozoa, and only one of these is needed for the 

 fertilisation of an ovum. 



Spermatogenesis. The spermatozoa result from the division of 

 the original germ cells, and the stages which have already been described 

 in our account of the structure of the testis may bo represented in a 

 diagrammatic form (see fig. 560). 



The germ cell divides into spermatogonia which undergo several 

 divisions, two of which are shown in the diagram. Each 

 spermatogonium in the end grows and becomes a primary spermato- 

 cyte ; it divides into two secondary spermatocytes, and each of these 

 into two spermatids which develop into spermatozoa. In the division 

 of the primary into the secondary spermatocytes, the mitosis is 

 heterotype and the number of chromosomes is reduced to half the 

 normal number. This phenomena is paralleled in the history of the 



