DEVELOPMENT OF THE DECIDUA. 651 



mucous membrane, their closed extremities resting on the subjacent 

 muscular tissue, while their mouths open into the cavity of the uterus. 

 A few fine blood-vessels penetrate the mucous membrane from below, 

 and, running upward between the tubules, encircle their superficial 

 extremities with a capillary network. There is no connective tissue 

 in the uterine mucous membrane, but only a few isolated nuclei and 

 spindle-shaped fibre-cells between the tubules. 



Decidua Vera. As the fecundated egg descends through the Fallo- 

 pian tube, the uterine mucous membrane takes on an increased activity 

 of growth. It becomes tumefied and vascular ; and, as it increases in 

 thickness, it projects, in rounded eminences, into the uterine cavity. 

 (Fig. 204.) The uterine tubules increase both in length and in width ; 

 and their open mouths become perceptible on the uterine surface, like 

 minute perforations. According to Kolliker, so early as the end of the 

 first week, they have increased to three or four times their original 

 size, measuring on the average 0.20 millimetre in diameter. The blood- 

 vessels of the mucous membrane also enlarge and communicate freely 

 with each other ; the vascular network between and around the tubules 

 thus becoming more extensive and abundant. The internal surface of 

 the uterus presents a thick, rich, soft, velvety, and vascular lining, quite 

 different from that found in the unimpregnated condition. It is now 

 known as the decidua ; and, in order to distinguish it from a similar 

 growth of subsequent formation, it has received the special name of the 

 decidua vera. 



The production of the decidua is confined to the body of the uterus, 

 the mucous membrane of the cervix taking no part in the process, but 

 retaining its original appearance. The decidual membrane commences 

 above, at the orifices of the Fallopian tubes, and ceases below, at the 

 os internum. The cavity of the cervix, meanwhile, is filled with an 

 abundant secretion of viscid mucus, which blocks up its passage, and 

 protects the internal cavity. If the uterus be opened in this condition, 

 its body will be seen lined with the decidua vera, which is continuous, 

 at the os internum, with the unaltered mucous membrane of the cervix. 



Decidua Reflexa. As the fecundated egg passes the lower orifice of 

 the Fallopian tube, "it insinuates itself between the opposite surfaces of 

 the uterine mucous membrane, and becomes lodged in one of the depres- 

 sions between the folds of the decidua. (Fig. 204.) At this situation 

 an adhesion subsequently takes place between the external membrane 

 of the egg and the uterine decidua. At the point where the egg thus 

 becomes fixed, there is a still more rapid development of the uterine 

 mucous membrane. Its projecting folds grow up around the egg in 

 such a manner as to partially enclose it in a kind of circumvallation, 

 and shut it off, in great measure, from the surrounding parts. (Fig. 205.) 

 The egg thus comes to be contained in a cavity of its own, which still 

 communicates for a time with the general cavity of the uterus, by an 

 opening over its most prominent part. As the process goes on, this 

 opening becomes narrower, while the decidual folds approach each other 



