THE MAMMALIAN SEXUAL CYCLE 43 



previously described. The uterine cavity contains liquid mucus 

 secreted by the glands, and this furnishes a medium in which 

 the spermatozoa swim. The mucous membrane of the uterus 

 is recuperated at those places where haemorrhage had occurred, 

 and external bleeding stops. The whole period lasts about 

 a week. 



Qilstrus in the bitch is followed either by jyregnancy or by 

 pseido-pregnancj/. The changes which occur in pregnancy in a 

 mammal are dealt with in the next chapter, but here it may be 

 remarked that as far as the internal tissues are concerned there 

 is a similarity between certain of the changes which take place 

 under the two conditions, only those of pregnancy are more 

 accentuated. 



In both pregnancy and pseudo-pregnancy the ovarian changes 

 are identical, at any rate in the earlier stages. At ovulation 

 the egg along with the follicular epithelial cells immediately 

 surrounding it and most of the liquor folliculi is discharged. 

 The remaining epithelial cells, which constitute the majority, 

 then undergo a rapid hypertrophy and become converted 

 eventually into the luteal cells of the fully formed corpus luteum. 

 At the same time strands of connective tissue grow inward from 

 the wall, and these contain blood vessels. Thus the luteal cells 

 are separated from one another by a network of connective tissue 

 and the whole structure is richly vascularised. What remains 

 of the central cavity of the original follicle is eventually filled 

 in by a plug of connective tissue. The process of formation is 

 extraordinarily rapid, only occupying a few days. The luteal 

 cells develop to at least sixteen times the cubical content of the 

 follicle cells, and a lipoid substance is formed within them. The 

 corpus luteum or " yellow body " which is thus formed from the 

 Graafian follicle after it has lost its ovum, persists throughout 

 pregnancy or pseudo-pregnancy, but in the latter stages of the 

 period it is in a state of regression. 



After parturition the corpus luteum degenerates still further, 

 and is eventually reduced to a mere scar. 



The description as above given of the process of formation 

 of the corpus luteum is based on that given by Sobotta for the 

 mouse and confirmed by various investigators for many other 

 mammals in which the phenomena are essentially similar. But 

 the details have not been worked out in the bitch. 



