THE PLACENTA. 



49 



Fig. 57. 



Main stalk 



maternal blood-spaces of the placenta. The active outgrowth of the mesoblastic 

 tissue of the chorion into the trophoblastic envelope results in the production of the 

 characteristic villous condition distinguishing the early human embryonic vesicle. 



When sectioned, the well-developed chorio7iic villi are seen to be composed of 

 two portions, {a) the central core of gelatinous connective tissue, containing nu- 

 merous stellate cells and blood-vessels, repre- 

 senting the foetal mesoblast, and ((5) the epi- 

 thelial covering derived from the trophoblast. 

 The investment of the villi consists of two 

 layers, — an inner stratum, next the connective- 

 tissue core, composed of low, distinctly out- 

 lined polyhedral cells, the chorionic epiihe- 

 lium, and an outer stratum, the syncytitim, 

 composed of an apparently continuous proto- 

 plasmic layer, in which nuclei are visible, but 

 definite cell boundaries are wanting. Irregu- 

 larly distributed aggregations of nuclei, or 

 cell-patches (Fig. 56), form slight elevations 

 on the surface of the villi. The derivation 

 of the outer layer, or syncytium, has been 

 the subject of much discussion ; its close rela- 

 tion to the maternal blood- spaces suggested a 

 maternal origin to some investigators, while 

 others regard it as a foetal production. The 

 observations of Peters on the very early human 

 ovum, already mentioned, conclusively show 

 the correctness of the latter view, and that 

 the syncytium is formed by the transformation of the trophoblast next the vascular 

 lacunae (Fig. 58) ; the syncytium, as well as the remaining parts of the villi of the 

 chorion, therefore, is of foetal origin. The epithelium covering the villi of the pla- 

 cental area early evinces a tendency towards regression, and by the fourth month 

 exists only as isolated patches ; during the later stages, and particularly on the 

 larger villi, the layer of chorionic epithelium disappears, the syncytium remaining 

 as the sole attenuated covering of the connective-tissue core of the villi. In certain 

 parts of its extent, especially where it covers the chorion and the decidua serotina, 



Isolated tuft of chorionic villi from placenta. 



X38. 



Fig. 58. 



Chorionic mesoblast 



Mesoblastic core of foetal 

 villus 



Trophoblast 

 Svncvtium 



'-<^ Maternal blood-space 



W -"^^Muscle 



Endothelium 

 Maternal blood-vessel 



Diagram showing formation of placenta. {Peters.) 



as well as upon some of the villi, the syncytium undergoes degeneration and is 

 replaced by a peculiar layer of hyaline refracting material known as cajialized 

 fibrin. 



The Placenta. — The placenta constitutes, from the third month of intra- 

 uterine life, the nutritive and respiratory organ of the foetus. As seen at birth, it is 

 of irregular discoidal form, concavo-convex in section, and measures from Jourteen 

 to eighteen centimetres in diameter and from three to four centimetres in thickness. 



4 



