DEVELOPMENT. 787 



It would seem that, at the villi of the placental tufts, where the 

 foetal and maternal portions of the placenta are brought into close rela- 

 tion with each other, the blood in the vessels of the mother is 

 separated from that in the vessels of the foetus by the intervention of 

 two distinct sets of nucleated cells (fig. 484). One of these (b) belongs 

 to the maternal portion of the placenta, is placed between the membrane 

 of the villus and that of the vascular system of the mother, and is prob- 

 ably designed to separate from the blood of the parent the materials 

 destined for the blood of the foetus; the other (/) belongs to the fcetal 

 portion of the placenta, is situated between the membrane of the villus 

 and the loop of vessels contained within, and probably serves for the 

 absorption of the material secreted by the other sets of cells, and for its 

 conveyance into the blood-vessels of the foetus. Between the two sets of 

 cells with their investing membrane there exists a space (d) , into which 

 it is possible that the materials secreted by the one set of cells of the 

 villus are poured in order that they may be absorbed by the other set, 

 and thus conveyed into a fcetal vessel. 



Not only, however, is there a passage of materials from the blood of 

 the mother into that of the foetus, but there is a mutual interchange of 





Fig. 484. Extremity of a placental villus. a, lining membrane of the vascular system of 

 the mother; 6, cells immediately lining a; d, space between the maternal and total portions of 

 the villus ; e, internal membrane of the villus, or external membrane of the chorion ; /, internal 

 cells of the villus, or cells of the chorion ; g, loop of umbilical vessels. (Goodsir. ) 



materials between the blood both of foetus and of parent ; the latter sup- 

 plying the former with nutriment, and in turn abstracting from it 

 materials which require to be removed. 



The placenta, therefore, of the human subject is composed of a 

 fcetal part and a maternal part, the term placenta properly including 

 all that entanglement of foetal villi and maternal sinuses, by means of 

 which the blood of the foetus is enriched and purified after the fashion 

 necessary for the proper growth and development of those parts which 

 it is designed to nourish. 



The whole of this structure is not, as might be imagined, thrown 

 off immediately after birth. The greater part, indeed, comes away at 

 that time, as the after-birth; and the separation of this portion takes 

 place by a rending or crushing through of that part at which its cohe- 

 sion is least strong, namely, where it is most burrowed and undermined 



