30 INTERNAL SECRETIONS OF THE OVARY 



disturbed by follicular atresia. The opossum is remarkable 

 among polyoestrous animals for the fact that pregnancy, which 

 lasts only about thirteen days, occurs without disturbing the 

 normal periodicity of oestrus. 



The ovarian cycle consists of the rapid growth of Graafian 

 follicles just before oestrus, and ovulation about one day later. 

 The uterine cycle is characterized by growth at oestrus, which 

 is continued during the pseudo-pregnant period or during 

 pregnancy. During anoestrus the uterus shows a reduction of 

 both the mucosaand themuscular layer. The glands are straight 

 or only very slightly coiled, while the lumen is small and the 

 epithelium has one layer. At prooestrus, when the Graafian 

 follicles have attained about a quarter of their maximum size, 

 the uterus undergoes distinct changes. The gross size becomes 

 greaterowing to increase in the various elements, to increased vas- 

 cularity, and, above all, to infiltration of lymph into the mucosa. 

 These changes begin in early prooestrus and culminate some days 

 after oestrus. By the time that oestrus sets in, the uterus is 

 considerably swollen and the glands greatly coiled. The 

 lumen has become much enlarged. After ovulation the growth 

 of the uterus continues, and is characterized by further hyper- 

 trophy of the mucosa and glands. At about the eleventh day 

 after ovulation, in the absence of pregnancy, the uterus under- 

 goes atrophy. The mucosa collapses owing to the withdrawal 

 of lymph, and the epithelial glands undergo degeneration. The 

 uterus then returns to the dioestrous condition in which the 

 epithelial glands consist of low columnar or cuboidal cells, and 

 degenerating material is found in the lumen. 



The vaginal cycle shows equally obvious changes. During 

 anoestrus, the epithelial lining is thin and there is no cellular 

 debris in the lumen. At prooestrus the vagina grows in diameter 

 and the epithelium becomes thickened. At the end of this stage 

 the mucosa is at least twelve to fifteen cells thick. No 

 leucocytes are to be seen, and the epithelium and the vaginal 

 smear consist of large, flat, nucleated cells. At oestrus these 

 nucleated cells are replaced in the vaginal smear by true 

 cornified cells which arise from the cornified mucosa of the 

 vagina. After ovulation, leucocytes begin to appear in 

 the vaginal smear, and within three or four days the smear 



