EXPERIMENTAL STUDIES ON FETAL ABSORPTION. 57 



placenta endotheliochorialis by Grosser (1909), from the fact that endothelium and 

 chorion form the line of juncture between the two tissues. Lastly, there is the type 

 of placenta common to Rodentia, Insectivora, Chiroptera, and Primates, in which 

 even the endothelium of the maternal vesselsis obliterated by thetrophoblast,sothat 

 the maternal blood flows into spaces bounded only by the fetal cells. Therefore, there 

 remain between the maternal and the fetal blood-streams solely the thin layer of 

 chorionic syncytium, the delicate stroma of the villi, and the endothelium of the 

 embryonic capillaries. This type has been termed by Grosser placenta hemochorialis. 

 The most careful study on the development of the placenta of the cat has been 

 made by Duval (1894), who showed that the trophoblast invades the uterine wall, 

 absorbing the uterine epithelium and the decidua, and forming (as it advances) 

 a mass of syncytium and giant cells which gradually invests the maternal vessels. 

 According to this observer, the syncytium and giant cells which form the bulk of 

 the placenta consist principally of chorionic ectoderm. One finds, therefore, 

 in the labyrinth of the mature placenta, numerous maternal capillaries, lined with 

 endothelium and surrounded by masses of epithelium of fetal origin, beneath which 

 lie the stroma of the villi and the embryonic blood-vessels. Duval's view that the 

 syncytium and the giant cells are of ectodermal origin in the cat and dog has been 

 generally accepted, but not without several noteworthy opponents. Thus Bonnet 

 (1903) claims that a narrow zone of decidua separates the chorionic ectoderm from 

 the maternal vessels. He believes, however, that late in pregnancy this strip of 

 decidua becomes intimately fused with the chorionic ectoderm and forms a lamellar 

 or syncytial layer in which the character of the individual elements is no longer 

 recognizable. Another view is held by Sehoenfeld (1903), who believes that de- 

 cidual cells contribute to the formation of the syncytium, but that the incorporation 

 of these cells does not prevent the chorionic epithelium trom coming into direct 

 contact with the maternal endothelium. 



A similar difference of opinion exists regarding the origin of the epithelial 

 masses which occur at the junction of the chorion and decidua basalis ("Umlager- 

 ungs-zone" of Strahl). Duval believes that "les lames basales" and "les arcades 

 ectodermiques," as he calls these structures, are composed of fetal ectoderm, whereas 

 Grosser believes they are largely decidual cells and hence should be designated 

 symplasma. 



Duval has pointed out that in the cat the chorionic ectoderm is composed of 

 cells with fairly well defined boundaries, whereas in the dog it is largely syncytial. 

 In the cat the chorionic ectoderm forms lamella? of epithelium about the maternal 

 capillaries, and in it two types of cells may be discerned. One finds large, oval 

 cells (termed by Duval central or giant cells) which lie close to the capillary endo- 

 thelium and which have a single nucleus or often two or three large round nuclei and 

 a vacuolated cytoplasm. These cells become more conspicuous as the placenta 

 matures, and Duval believes that they arise from the less differentiated epithelium 

 beneath them. Some have thought that they are of maternal origin and have desig- 

 nated them decidual or serotina cells. 



Surrounding the giant cells are numerous smaller cells with single, more deeply 

 staining nuclei. These at first possess abundant cytoplasm and distinct cell 



