PLACENTA 73 



In recent years several early ova l imbedded in the mucous membrane have 

 been studied by modern histological methods, and the facts elicited have con- 

 siderably modified the older views as to the structure of the placenta, and the 

 interrelations of uterine and foetal tissues in it. 



In the earliest known stage, the ovum, as already stated, lies imbedded in the 

 stratum compactum of the mucosa. There is good reason for believing that it has 

 eaten its way into the mucous membrane by the activities of its trophoblastic 

 covering. In Peters' ovum (figs. 93 and 98) and the still earlier ovum of Leopold, 

 the trophoblast is composed of irregular cellular strands attached by their outer ends 

 to the decidua. In the interspaces between these are lacuna) filled with maternal 

 blood. The trophoblast is bounded on the foetal side by a fairly regular epithelial 

 layer (fig. 99, ect), which is lined by mesoderm. It is indented at intervals by the 

 processes of that layer, which become the cores of the future villi. The blood -lacun-de 

 reach down to this inner layer. They are everywhere lined by a nucleated proto- 

 plasmic lamella in which there are no traces of cell-outlines. This is known as the 

 placental plasmodium or syncytium. The plasmodium where it lines the blood- 

 lacunae is reduced to a thin endothelium-like layer, but where the decidua and 

 trophoblast merge it spreads out into masses, which are seen invading the 

 decidua and pushing their way into the capillaries. The stratum compactum is 

 beginning to be crowded with decidual cells, the existing capillaries are enormously 

 enlarged, and there is evidence of the formation of new blood-channels. Immedi- 

 ately round the ovum there is a zone in which the decidual changes are taking place 

 more actively ; it contains many large decidual cells, leucocytes and extravasated 

 red blood-corpuscles, besides multinucleated elements of which it is difficult to 

 affirm whether they are foetal or maternal derivatives (fig. 99). Some certainly, 

 probably all, are of trophoblastic origin. In this zone the capillaries are dilated 

 and sinus-like, and open directly into the blood-lacunae. Masses of syncytium 

 are seen in the interior of capillaries, and many sections show capillaries which 

 are lined on the decidual side by endothelium, and on the side of the ovum by 

 syncytium (fig. 99, sy 2 ). That the vessels are being opened up, and the endothelium 

 destroyed by the trophoblast, is clearly indicated by these appearances, as well as by 

 the occurrence of masses of broken-down endothelial cells. 



We do not know by direct observation how this stage is reached in the case of 

 the human ovum, but recent work on the comparative histology of the placenta 

 leaves no room for reasonable doubt on the main point viz. that cellular strands, 

 syncytial masses, and syncytial lining of the blood-lacunae are all equally 

 derivatives of the chorionic ectoderm. 



Origin of the placental syncytium. The view here adopted that the syncytium is 

 merely the surface-layer of the chorionic epithelium is now very generally accepted, but there 

 are some other interpretations of the appearances which may be briefly alluded to. 



1. It has been derived by some from the maternal epithelium either of the surface or of 

 the glands of the mucosa. If the newer views as to the imbedding of the ovum be correct, 

 such a derivation is very improbable ; but it is finally excluded by the fact that the villi of a 

 chorionic vesicle, which has become imbedded in the ovary owing to fertilisation of the ovum 

 having occurred while it was yet in the Graafian follicle, are provided with a well developed 

 l>lasm<nlial layer. Fig. 100 is a drawing of a villus branch from such a case, and if compared with 

 fig. 104, p. 76, which is a drawing of a villus from an early uterine pregnancy, it will be seen that 

 the mesodermic core is covered, in both, by identical layers cellular and syncytial. 2 



1 See H. Peters, Ueber die Einbettung des menschlichen Eies, &c. Deuticke, Leipzig und Wien, 

 l.s'.m ; Si.g,.nbeck van Heukelom, Arch. f. Anat. 1898 ; Marchand, Arch. Gyniikol. 1904 : Anat. Anzeiger 

 (Erganzungsheft), xxi. : Anat. Hefte, H. 67, xxi. ; Rossi Doria, Arch. Gyniikol. Ixxvi. ; Beneke, Deut. 

 mod. Wochenschr. Jahrg. xxx. 1904 : Monatschr. f. Geburtsh. u. Gyniikol. xix. ; Graf v. Spee, Verhandl. 

 deutsch. Ges. Gyniikol. xi. 1905 ; Leopold, Arbeit, a. d. k. Frauenklinik, Dresden, iv. ; H. Happe, Ar>at. 

 xxxii. ; Keibel, Anat. Anz. (Ergiinzungsheft>, xxx. 1907 ; Frassi, Arch. f. mikr. Anat. Ixx. 1907. 



Ovarian pregnancies are very rare. I am indebted to Dr. Munro Kerr for the opportunity of 

 examining the sections (prepared by Dr. J. H. Teacher) of an ovary, fixed immediately after removal 



