974 OF GENERATION. 



the ovum enters the uterus, it becomes partially imbedded in the substance of 

 the decidua, which is as yet quite soft (Fig. 251) ; and this, receiving an in- 

 creased nutrition at the part with which the ovum comes into contact, grows up 



around it, very much after the man- 

 Fig. 252. ner in which the fleshy granulations 

 grow up around the pea imbedded in 

 a caustic issue. This extension of 

 the decidual substance continues (Fig. 

 252), until i* nas completely enve- 

 loped the ovum; and it is thus, ac- 

 cording to him, that the decidua re- 

 flexa is formed, in continuity with 

 the decidua vera. 1 As the ovum 

 increases in size, the cavity between 



'iSBH^^ the decidua vera and the decidua re- 



flexa gradually diminishes; and by 



More advanced stage of the same. the end of the third month the two 



layers come into contact, and are 

 henceforth scarcely or not at all distinguishable. 



979. The surface of the Ovum, thus surrounded by the double layer of the 

 deciduous membrane, is rendered shaggy by the growth of villous tufts from 

 the surface of its investing chorion (Plate I. Fig. 10, /, /). Each of these 

 tufts, as was first pointed out by Prof. Groodsir, 2 is composed of an assemblage 

 of nucleated cells, which are found in various stages of development ; and these 

 are always inclosed within a layer of basement-membrane, which seems to be 

 itself composed of flattened cells united by their edges. At the free extremity 

 of each villus is a bulbous expansion, the cells composing which are arranged 

 round a central spot ; and it is at this point that the most active processes of 

 growth take place, the villus elongating by the development of new cells from 

 its germinal spot, and (like the spongiole of the plant) drawing in nutriment 

 from the soil in which it is imbedded. In its earliest grade of development, 

 the chorion and its villi contain no vessels ; and the fluid drawn in by the tufts 

 is communicated to the embryo, by the absorbing powers of the germinal mem- 

 brane of the latter. But when the tufts are penetrated by bloodvessels, and 

 their communication with the embryo becomes more direct, the means by which 

 they communicate with the parent are found to be still essentially the same ; 

 namely, a double layer of nucleated cells, one layer belonging to the foetal tuft, 

 and the other to the vascular maternal surface. It is from these elements that 

 the Placenta is formed, in the manner next to be described. 



980. The first stage in this process consists in the extension of the foetal 

 vessels into the villi of the Chorion over its entire surface, in the manner here- 

 after to be detailed ( 1001); so that the nutriment which these villi imbibe, 

 instead of being merely added to the albuminous fluid surrounding the yelk- 

 bag, is now conveyed directly to the embryo. This the earliest and simplest 

 mode by which the Foetus effects a new connection with the parent is the only 

 one in which it ever takes place in. the lower Mammalia, which are hence pro- 

 perly designated as " non-placental," rather than as ovo-viviparous. In the 

 higher Mammalia, however, there soon occurs a great extension of the vascular 



1 This doctrine was first announced by M. Coste, in a communication to the Parisian 

 Academy of Sciences, on the basis of observations on two Uteri at the 20th and 25th days 

 of gestation. (See "Comptes Rendus," Mai 24, 1847.) It seems to be altogether that 

 which is most in harmony with observed facts; and especially with those which have been 

 noticed by Professors Weber and Sharpey. See, also, the Memoir of M. Robin, on the 

 Mucous Membrane of the Uterus, in the " Archiv. Gen. deMed.," 4e Ser., torn. xvii. xviii. 



2 "Anatomical and Pathological Observations," chap. ix. 



