FORMATION OF THE DECIDUA. 



619 



Fig. 222. 



formation of the embryo in the egg, and the development of the 

 amnion and chorion have been going on simultaneously. Soon 

 after the entrance of the egg into the uterine cavity, its external 

 investing membrane becomes covered with projecting filaments, or 

 viliosities, as previously described. (Chap. X.) These viliosities, 

 which are at first, as we have seen, solid and non-vascular, insinuate 

 themselves, as they grow, into the uterine tubules, or between the 

 folds of the decidual surface with which the egg is in contact, pene- 

 trating in this way into little cavities or follicles of the uterine 

 mucous membrane, formed either from the cavities of the tubules 

 themselves, or by the adjacent surfaces of minute projecting folds. 

 When the formation of the decidua reflexa is accomplished, the 

 chorion has already become uniformly 

 shaggy ; and its viliosities, spreading in all 

 directions from its external surface, pene- 

 trate everywhere into the follicles above de- 

 scribed, both of the decidua vera underneath 

 it and the contiguous surface of the decidua 

 reflexa with which it is covered. (Fig. 222.) 

 In this way the egg becomes entangled 

 with the decidua, and cannot then be read- 

 ily separated from it, without rupturing 

 some of the filaments which have grown 

 from its surface, and have been received 

 into the cavity of the follicles. The nu- 

 tritious fluids, exuded from the soft and 

 glandular textures of the decidua, are now 

 readily imbibed by the viliosities of the chorion ; and a more rapid 

 supply of nourishment is thus provided, corresponding in abun- 

 dance with the increased and increasing size of the egg. 



Very soon, however, a still greater activity of absorption be- 

 comes necessary ; and, as we have seen in a preceding chapter, the 

 external membrane of the egg becomes vascular by the formation 

 of the allantoic bloodvessels, which emerge from the body of the 

 foetus, to ramify in the chorion, and penetrate everywhere into the 

 viliosities with which it is covered. Each villosity, then, as it lies 

 imbedded in its uterine follicle, contains a vascular loop through 

 which the foetal blood circulates, increasing the rapidity with which 

 absorption and exhalation take place. 



Subsequently, furthermore, these vascular tufts, which are at first 

 uniformly abundant throughout the whole extent of the chorion, 



IMPREGNATED UTERUS: 

 showing connection between vil- 

 iosities of chorion and decidual 



membranes. 



