HISTOLOGY OF THE HUMAN CH ORION. 363 



it (cf. especially page 357 and Fig. 238); it represents only a portion of the de- 

 cidua, the other portion having remained upon the uterine wall. The spongy mass 

 is found upon examination to consist of an immense number of tufts of fine rods 

 of tissue, which are irregularly cylindrical in shape. Further examination shows 

 that they are twigs (Fig. 248) with rounded ends and springing from branchlets 

 which in their turn arise from branches, and so on until a large main stem is 

 found, which starts from the chorion. This branching system is richly supplied with 

 blood from the fetal vessels on the surface of the placenta. The villi are inter- 

 woven 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" (Vol. V, page 716), in reference to the placental 

 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 embedded 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 decidua 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. 241, 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 interrupted here and there. Subsequent 

 writers have gone but little beyond Farre's account, which has been entirely over- 

 looked 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 

 Bouin's picro-formalin fluid. Pieces may be stained in toto with alum cochineal 

 or borax carmine and transverse sections cut in paraffin. The sections may be 

 advantageously counterstained with eosin or orange G. 



For the general history of the chorion see page 82. As it is formed by the 

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

 latter comprises mesenchyma and mesothelium. 



