CHORION. 



"47 



Fig. 261. 



portion afterward becomes, in the process of further development, the 

 urinary bladder. The rounded cord or "urachus," which, in the adult, 

 runs from the superior fund us of the bladder to the situation of the 

 umbilicus in the abdomen, is the vestige of the obliterated canal of the 

 primitive chorion. 



The next peculiarity of the chorion is, that it becomes shaggy. Even. 

 while the egg is still very small, and has but recently found its way into 

 the uterine cavity, its exterior is already covered with transparent villi 

 (Fig. 259), which increase the extent of its surface, and, assist in the 

 absorption of fluids from without. The villi are at this time quite sim- 

 ple in form, and homogeneous in structure. 



As the egg increases in size, the villi elongate, and become ramified 

 by the repeated budding of lateral offshoots. After this process has 

 continued for some time, the outer 

 surface of the chorion presents a uni- 

 formly shaggy appearance, owing to its 

 being covered everywhere with com- 

 pound villosities. 



The villosities, when examined by 

 the microscope, have an exceedingly 

 characteristic appearance. They origi- 

 nate from the surface of the chorion by 

 a somewhat narrow stem, and divide 

 into secondary and tertiary branches 

 of varying size and figure; some of 

 them filamentous, others club-shaped, 

 many of them irregularly swollen at 

 various points. All terminate by 

 rounded extremities, giving to the 

 whole tuft a certain resemblance to 

 some varieties of sea-weed. The larger 

 trunks and branches of the villosity 

 are seen to contain minute nuclei, im- 

 bedded in a nearly homogeneous, or 

 finely granular substratum. The 



smaller ones appear, under a low magnifying power, simply granular in 

 texture. 



The villi of the chorion are quite unlike any other structure to be 

 met with in the body. Whenever we find, in the uterus, any portion of 

 a membrane having villosities of this character, it is certain that preg- 

 nancy has existed ; for such villosities can only belong to the chorion, 

 and the chorion itself is a part of the foetus. The presence of portions 

 of a shaggy chorion is therefore as satisfactory proof of the existence 

 of pregnancy, as if the body of the foetus itself had been found. 



While the villosities just described are in process of formation, the 

 chorion receives a supply of bloodvessels from the interior of the body 

 of the embryo. The arteries, which are a continuation of those dis- 



Compound villosity of the HUMAN 

 CHORION, ramified extremity. From 

 a three months' foetus. Magnified 30 

 diameters. 



