74 PLACENTA 



2. The outer covering of the villi has been regarded as derived from the decidual tissue, 

 or from the endothelium of the maternal capillaries. The idea is that there occurs a blending 

 or interlocking of foetal (chorionic) and maternal (decidual) tissue, so that the villi become clothed 

 with a layer of decidual tissue (subchorionic membrane, Turner), or that the endothelium of 

 the dilated capillaries persists as the lining of the blood-spaces (Waldeyer). Such an inter- 

 pretation is difficult to disprove directly, but it is inconsistent with the newer views regarding 

 the nature and the activities of the trophoblast. Recent research has provided nearly con- 

 clusive evidence for the simpler reading of the facts, advanced more especially by Hubrecht and 

 Van Beneden for lower mammals, and for the human subject by Peters, Leopold, Minot, Webster, 

 Hart and Gulland, and others, in terms of which the whole placenta, except a thin layer of 

 decidua on its uterine surface, is derived from the chorion that, in short, it is a great sponge- 

 like mass of foetal tissue filled with maternal blood. 



If we proceed, then, upon the assumption that the cellular strands (cytdblast, 

 Van Beneden) and the syncytium (plasmodiblast, Van Beneden) are both derivatives 



FIG. 100. SECTION OF A VILLUS FROM A CASE OF OVARIAN PREGNANCY. (T. H. Bryce.) 

 sy, syncytium ; L.I., Langhans' layer. 



of the chorionic epithelium, the appearances described in Peters' and Leopold's 

 ova may be explained as follows : At a very early stage, before there is any 

 differentiation of the embryonic ectoderm, the trophoblast proliferates actively, 

 and its surface becomes irregular by the outgrowth of epithelial buds. As these 

 extend into the decidua the maternal tissue is absorbed before them.' 2 The 

 capillaries are opened up by the destruction of their endothelial walls, and the 

 maternal blood is thus extravasated into the spaces between the strands of the 

 trophoblast. These spaces now necessarily form a system of intercommunicating 

 blood-lacunae (fig. 98). 



by operation, in which a blastocyst lay imbedded partly in the ovarian stroma and partly in a mass 

 of fibrin and extravasated blood. The evidence of the sections is conclusive in favour of the foetal 

 origin of both cellular and syncytial layers. T. H. B. 



1 The trophoblast-cells are supposed to act like phagocytes, and the maternal tissue to serve as 

 pabulum for the growing ovum. It was on account of the assumed physiological activity of the 

 chorionic epithelium that Hubrecht gave it the name of ' trophoblast ' i.e. ' trophic epiblast.' Here, 

 and elsewhere in this work, the term ' trophoblast ' is used in Hubrecht's original sense, to signify that 

 part of the ectoderm which does not share in the formation of the embryo, but constitutes the wall of 

 the blastocyst. Minot has introduced the word troplioderm for the proliferated chorionic ectoderm, or 

 mantle, which is concerned in the implantation of the egg. The term is not used in this work, because 

 trophoblast has the priority, and also because in some particulars the processes involved are here 

 interpreted rather differently than by Minot (Trans. Amer. Gynec. Soc. 1904). 



