EELATIOXS OF THE DECIDUA. ' 715 



from an inch to an inch and a half, and the nterus has nearly donbled 

 the size which ii presents in the nnimpregnated state, the swelling formed 

 by the ovum and decidua projects strongly like a tumour within the 

 uterine cavity. The membrane of the other parts of the uterus has 

 also undergone progressive increase in its thickness by decidual hyper- 

 trophy, so that, having become, as it were, too wide for the capacity 

 of the cavitA', it is thrown into a number of grooves enclosing irregular 

 folds and mammillary projections, but still exhibiting throughout the 

 peculiar features of the mucous lining membrane. 



On the side of the ovum which is towards the uterine wall, there is also 

 a layer of decidua, in which the villi of the chorion are imbedded. At 

 this place, from the sixth to the eighth week, these villi begin to be more 

 thickly set and of larger size, aud to undergo a more complex ramifica- 

 tion than on the other sides; and as at the same time there is a correspond- 

 ing increase in the decidual substance, these villi become more and more 

 closely involved in it, and there is thus established the commencement 

 of that more intimate combination of foetal villi and decidual substance, 

 which by its progressive development in the two or three following 

 weeks, gives rise to the formation of the placenta. 



By the clianges now described there has become apparent the dis- 

 tinction of the three portions of decidua usually recognised by authors, 

 viz., decidua vera, decidua reflexa, and -decidua serofi/ia. The first of 

 these is that portion of the altered membrane which lines the general 

 cavity of the uterus in every part except that occupied by the attach- 

 ment of the ovum ; the decidua reflexa is that w'hich covers the ovum 

 as it projects into the uterine cavity, and which is continuous with the 

 decidua vera at the base of the swelling. 



The name of decidua serotina has been somewhat variously employed 

 by authors ; but may, in the meantime, be most suital)ly applied to the 

 whole of the decidual substance inteiwening between the ovum and the 

 uterus, and which may include, therefore, both that which is concerned in 

 the formation of the placenta, and the distinct layer of decidual sub- 

 stance which at a later period is found covering the uterine surface of 

 the placenta. 



The cavil u of ilie decidua, which intervenes between the decidua vera 

 and decidua reflexa, and which subsists during the first half of the 

 period of pregnancy, is obviously the same as the original uterine 

 cavity, and, so long as it remains open, naturally communicates with 

 the Fallopian tubes at the upper angles, and the canal of the cervix 

 at the lower. In the last three months of pregnancy, however, this 

 cavity is completely obliterated by the union of the decidua vera and 

 reflexa into one layer over the whole of their extent, so that when 

 afterwards much extended and I'educed to a comparatively thin and 

 irregular stratum of substance, they are at birth thrown off as one 

 membrane along with the other envelopes of the foetus. 



x\s the human ovum has never been observed in the progress of its 

 mcapsulation, the exact manner in which this occurs is still involved in 

 doubt. From the various observations, however, already referred to on 

 early ova which have undergone recent incapsulation, and the know- 

 ledge of what occurs in animals, it may be conjectured, as first suggested 

 by ^Sharpey (Baly's transl. of "Mailer's Physiology," 1842, p" 1580) 

 that the minute ovum when it arrives in the uterus may be sunk or 

 imbedded in the soft or spongy substance of the mucous membrane, and 



