CHAP, ii.] PREGNANCY AND BIRTH. 1513 



new growth does not give way to hsemorrhage and immediate 

 decay ; it 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 

 already undergone some developmental changes, is received; and 

 in this it becomes embedded, the new growth closing in over it. 

 Meanwhile the rest of the uterine structures, especially the 

 muscular tissue, become also much enlarged; as pregnancy ad- 

 vances a large number of new muscular fibres are formed. 



As the ovum, now developing into the embryo and its ap- 

 pendages, 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 rapidly developing 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 pro- 

 jecting ovum. That part of the decidua which intervenes between 

 the ovum and the nearest uterine wall is spoken of as the decidua 

 serotina. As the embryo with its appendages continues to en- 

 large, carrying with it the decidua reflexa, the latter becomes 

 pushed against the decidua vera, gradually obliterating the cavity 

 of the uterus, except at the cervix ; about the end of the third 

 month, in the human subject, the two come into complete contact 

 all over, and ultimately the distinction between them is lost. 



The changes through which the mucous membrane becomes 

 the decidua are seen in their simplest form in the decidua vera at 

 some little distance from the ovum, on the sides for instance of 

 the uterus. When this portion of the decidua is examined, it is 

 found that the glands have become very much enlarged and tor- 

 tuous with irregular lateral bulgings. The change is greatest in 

 the middle third of the length of the glands ; this region is seen 

 in sections to form a zone in which the channels of the glands (the 

 epithelium lining which is now cubical rather than columnar and is 

 devoid of cilia) appear as a number of irregular spaces giving the 

 zone a spongy texture. The mouths and necks of the glands 

 though enlarged and altered are not so markedly changed, and 

 the basal portions have undergone still less transformation. The 

 dermic connective tissue between the glands has also become 

 hypertrophied, though not in the same proportion as the glands ; 

 it is very vascular, and a number of new cells make their appear- 

 ance in it. 



Similar changes take place in the decidua reflexa, account 

 being taken of the fact that here the glands are by the intercala- 

 tion of the growing ovum cut off as it were from their bases. The 

 changes, however, undergone by the decidua reflexa, and by the 

 decidua vera in general, are of subordinate interest compared 

 with those which take place in that part of the decidua vera 



