STRUCTURE AND FORMATION OF THE PLACENTA. 721 



longed from the external decidiia serotina bet-^veen the lobes. (For an 

 excellent account of the evidence in favour of the foregoing views, 

 supported by original observations, see Professor Turner's observations 

 on the " Structure of the Human Placenta," in Proceed. Eoy. Sec. of 

 Edin., May, 1872, and in Journ. of Anat., vol. vii., p. 120.) 



Farther Consideration of the Structure of the Placenta. — Two doubtful 

 points respecting the structure of the placenta still requne consideration, viz.. 

 1st, the extent to which uterine tissue is included in or penetrates into, or 

 I'emains as a constituent of the maternal part ; and 2nd, the relation of the 

 interpenetration of the fojtal villi and the uterine decidua to the glandular or 

 other structures of the uterus. 



In regard to the first of these points, the views of anatomists still differ gi'eatly ; 

 for, on the one hand, some hold that there is no vestige of iiterine tissue left in 

 connection with the maternal blood-spaces, at all events in the deeper two-thirds 

 of the thickness of the placenta, and that consequently the maternal blood cii-cu- 

 lating in the iDlacenta is in direct contact with the epithelial covering of the 

 foetal villi ; while others are inclined to regard that epithelial covering, or some 

 part of the structui-e which appears to belong to the foetal villi, as really 

 containing some of the elements of the decidua. 



Goodstr, indeed, described a double cellular covering of the placental villi, 

 regarding the external layer as of uterine, and the internal as of foetal origin. 

 But later anatomists have not succeeded in confirming these observations, and it 

 does not appear certain that there is more than one obvious layer of cells over 

 the surface of the foetal villi. If, therefore, we assume the existence of a 

 layer of uterine cells in the fully formed placenta, we are reduced to the 

 necessity of supposing that it has either replaced that of the chorionic villi, or 

 has become closely incorporated with it (KoUiker). 



The observation of the gradual penetration of the decidua by the villi in the 

 earlier stages of placental formation, the possibility of separating the foetal and 

 maternal structui'es from each other during a certain time, and the undoubted 

 presence both of decidual tissue and of uterine blood-vessels possessed of walls 

 of their own in the commencement, — aU supply convincing proof that uterine 

 elements of structure have originally existed in the placenta, and have contri- 

 buted to its formation along with those derived from the fostus ; but the con- 

 dition of the uterine elements in the more advanced stages of placental gi'owth, 

 if they really then exist, still requires further investigation. 



The actual enlargement of the uterine capillaries of the decidua into the form 

 of vascular spaces has been traced by Virchow (Archiv, vol. iii. p. 450), and 

 Priestley has observed the capillary form of the maternal vessels surrounding the 

 villi in a product of abortion of the 8th week (Lectures on the Development of 

 the Gravid Uterus, 1860, p. 02). Some anatomists, indeed, as Schroder Van der 

 Kolk, afiirm that they have been able to detect the remains of a vascular wall in 

 connection with the blood-spaces of some parts of the placenta (Waamemingen 

 over het Maaksel van de Menschelijke Placenta : Amsterdam. 18G1), so that it 

 may with reason be surmised that the change which takes place in the stnictirre 

 of the parts forming the human placenta, in the course of the fourth month, is 

 mainly of the nature of a rapid thinning and absorption of the elements of the 

 decidual tissue and vascular walls. 



The view that the placenta originally consists of uterine as well as foetal ele- 

 ments combined receives the fullest confirmation from the study of the comparsr 

 tive anatomy of the various forms of simple placental structures in animals, as, 

 in the diffyge placenta of the pachydermata, solipeds, cetacea and some other 

 animals, and in the cotylcdonous placenta of most ruminants ; in both of 

 which only the foetal part of the placental structure undergoes separation from the 

 uterus at birth, — constituting the iwn-dccidnate iorm. of placentation ; and in the 

 various forms of more complete union of the foetal and maternal elements, which 

 occur in the zonal placenta of the camivora, and the discoid of the rodentia, in 

 which more or less of the maternal stmcture derived from decidual formation 

 comes away with the foetal product at birth. But in all forms of placentation of 

 A'ur.. ir. 3 a 



