CHAP, ii.] IMPREGNATION. 673 



As the result of the action of the spermatozoa on the ovum, the 

 latter, instead of dying as when impregnation fails, awakes to great 

 nutritive activity accompanied by remarkable morphological 

 changes ; it enlarges and develops into an embryo. No sooner, how- 

 ever, have these changes begun in the ovum than correlative changes, 

 brought about probably by reflex action, but at present most 

 obscure in their causation, take place in the uterus. The mucous 

 membrane of this organ, whether the coitus resulting in im- 

 pregnation be coincident with a menstrual period or not, becomes 

 congested, and a rapid growth takes place, characterized by a rapid 

 proliferation of the epithelial and subepithelial tissues. Unlike the 

 case of menstruation, however, this new growth does not give way 

 to immediate decay and haemorrhage, but remains; and may be 

 distinguished as a new temporary lining to the uterus, the so-called 

 decidua. Into this decidua the ovum, on its descent from the 

 Fallopian tube, in which it has probably already undergone some 

 developmental changes, is received; and in this it becomes em- 

 bedded, the new growth closing in over it. Meanwhile the rest of 

 the uterine structures, especially the muscular tissue, become also 

 much enlarged; as pregnancy advances a large number of new 

 muscular fibres are formed. As the ovum continues to increase in 

 size, it bulges into the cavity of the uterus, carrying with it the 

 portion of the decidua which has closed over it. Henceforward, 

 accordingly, a distinction is made in the now well-developed 

 decidua between the decidua reflexa, or that part of the membrane 

 which covers the projecting ovum, and the decidua vera, or the 

 rest of the membrane lining the cavity of the uterus, the two being 

 continuous round the base of the projecting ovum. That part of 

 the decidua which intervenes between the ovum and the nearest 

 uterine wall is frequently spoken of as the decidua serotina. As 

 the ovum developes into the foetus with its membranes, the decidua 

 reflexa becomes pushed against the decidua vera; about the end 

 of the third month, in the human subject, the two come into com- 

 plete contact all over, and ultimately the distinction between them 

 is lost. In the region of the decidua serotina the allantoic vessels 

 of the foetus develope a placenta. For an account of the various 

 changes by which these events are brought about, as well as of the 

 history of the embryo itself, we must refer the reader to anatomical 

 treatises. 



IT. 43 



