268 



EMBRYOLOGY. 



with numerous secondary sprouts and were clothed in a two-layered 

 chorionic epithelium, already attached by their tips in the maternal 

 tissue (attachment villi), and also the intervillous spaces filled with 

 maternal blood. But this was distinctly separated from the chorionic 

 epithelium by a special thin cellular membrane (E). This membrane 

 consisted of very thin endothelial cells, and was frequently elevated 

 more or less from the chorionic villi, probably owing to the method 

 of preparation. KEIBEL justly concludes from the establishment 

 of the existence of an endothelial membrane that the intervillous 

 spaces are the enormously dilated maternal capillaries. 



Between the chorionic epithelium and the walls of the maternal 



chorionic blood-vessels. 



' chorionic epithelium. 



' -.iiaternal endotheliv.m. B. 



TV-" 



Fig. 15D. Diagram of the structure of the human placenta from an embryo four wesks old, 

 after KEIBEL. 



Z, Chorionic villi ; Sp, attachment of the tips of the same in the maternal decidua (D) ; C, en- 

 larged maternal blood-capillaries. 



capillaries KEIBEL finds no further remnant of maternal tissue in 

 the very young ovum. This would indicate an early and complete 

 disappearance of the uterine epithelium, and would make it probable 

 that the protoplasmic layer and the canalised fibrin described at 

 p. 261 are to be derived from the cell-layers of the chorion, a mooted 

 point concerning which I have been unable to form a definite opinion. 

 Thus the observations are increasing which favor a special limita- 

 tion of the intervillous spaces and the existence of a thin layer of 

 maternal tissue, a vascular endothelium, upon the villi. 



6. The Umbilical Cord. 



The umbilical cord (funiculus umbilicalis) constitutes the union 

 between the placenta and the embryonic body (fig. 143). It is a cord 



