HISTOLOGY OF THE HUMAN CH ORION. 341 



villi are interwoven so that the twigs of one branch are interlaced with those of 

 another, and apparently separate twigs may grow together and their vessels 

 anastomose; but on this point we are unable to speak positively. The villous 

 twigs next the surface of the decidua penetrate that tissue a slight distance. 



The intervillous spaces are filled, or nearly so, with blood; they form a com- 

 plex system of channels. The intervillous blood is maternal. Farre says, in his 

 article in Todd's "Cyclopaedia" (V. Suppl., page 716), in reference to the pla- 

 cental decidua : ' ' Numerous valve-like apertures are observed upon all parts of 

 the surface; they are the orifices of the veins, which have been torn off from the 

 uterus. A probe passed into any one of these, after taking an oblique direction, 

 enters at once into the placental substance. Small arteries about half an inch in 

 length are also everywhere observed imbedded in this layer. After making 

 several sharp spiral turns they likewise suddenly open into the placenta"; and 

 on page 719 he adds : "These venous orifices occupy three situations. The first 

 and most numerous are scattered over the inner side of the general layer of de- 

 cidua which constitutes the upper boundary of the placenta; the second form 

 openings upon the sides of the decidual prolongations or dissepiments which 

 separate the lobes (cotyledons) from each other; while the third lead directly 

 into the interrupted channel in the margin, termed the circular sinus." The 

 circular sinus (Fig. 197, Si) is merely a space at the edge of the placenta which is 

 left comparatively free from the villi. It is not a continuous channel, but is inter- 

 rupted here and there. Subsequent writers have gone but little beyond Farre's 

 account, which has been entirely overlooked by most recent investigators, who, 

 accordingly, have announced as new discoveries many facts known to Farre. 

 Under these circumstances it is interesting to direct renewed attention to Farre's 

 masterly article. 



Histology of the Human Chorion. 



The chorion may be preserved in Zenker's or Tellyesnicky's fluid or in 

 Kleinenberg's picro-sulphuric acid. Pieces may be stained in toto with alum 

 cochineal or borax carmine and vertical sections cut in paraffin. The sections 

 may be advantageously counterstained with eosin or orange G. 



For the general history of the chorion see page 78. As it is formed by the 

 somatopleure, it comprises an outer ectoderm and an inner mesoderm, which 

 latter comprises mesenchyma and mesothelium. 



The ectoderm undergoes a very precocious growth producing a very large 

 number of cells, which form the thick trophoblastic layer as described on page 

 342. Then follows the stage in which, by degeneration, spaces are produced in 

 the trophoblast into which the blood of the mother enters and circulates; and at 



