FORMATION OF THE PLACENTA. 



905 



rion, dipping down into it. But in the Human placenta, the two elements 

 are mingled together through its whole substance. On looking at the foetal 

 surface of the Human placenta, we perceive that the umbilical vessels diverge 

 in every direction from the point at which they enter it ; and their subdivisions 

 form a large mass of capillaries, arranged in a peculiar manner (Figs. 320, 

 321), and constituting what are known as the foetal villi. Each villus contains 

 one or more capillary loops, communicating with an artery on one side, and 

 with a vein on the other; but the same capillary may pass into several villi, 

 before re-entering a larger vessel. The capillaries of the villi are covered, as 

 in the choriou, by a layer of cells (Fig. 321, a, a; Fig, 322, e), inclosed in a 

 basement-membrane; but the foetal tuft thus formed is inclosed in a second 

 series of envelopes (Fig. 322, a, c, e), derived from the maternal portion of 

 the placenta, a space (ips*) being left, however, between the two, at the ex- 

 tremity of the tuft. 



755." Whilst the foetal portion of the Placenta is thus being generated by 

 the extension of the vascular tufts of the Chorion, the maternal portion is 

 formed by the enlargement of the vessels of the Decidua, between which 

 they dip down. "These vessels assume the character of sinuses ; and at last 



FIG. 322. 



V, Plaeental Villus, consisting of gelatinous connective tissue with corpuscles; in its interior are 

 seen the festal bloodvessels forming loops ; e, e, epithelium investing the Villus, beneath which is a 

 layer of cells forming the occluding lamina; d, s, decidua serotina; c, a, curling artery ; u s, uterine 

 sinus; t, I, trabecula ; ips, intra-placental maternal blood sinus. 



swell out (so to speak) around and between the villi; so that finally the villi 

 are completely bound up or covered by the membrane which constitutes the 

 walls of the vessels, this membrane following the contour of all the villi, 

 and even passing to a certain extent over the branches and stems of the 

 tufts. Between this membrane, or wall of the enlarged decidual vessels, 

 and the internal membrane of the villi, there still remains a layer of the 

 cells of the decidua." * In this manner is formed the maternal portion of the 



1 Prof. Goodsir's Anatomical and Pathological Observations, p. 60. Prof. Turner 

 (Journ. of Anat. and Physiology, 1873, p. 130) has recently demonstrated, however, 



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