MEMBRANES OF THE IMPREGNATED EGG. 



647 



FIG. 200. 



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

 into the uterus, its exterior is covered with villi (Fig. 198), which 

 increase its extent of surface, and assist in the absorption of fluids 

 from without. The villi are at this 

 time simple in form, and homogeneous 

 in structure. 



As the egg increases in size, the 

 viTli elongate, and become ramified by 

 the repeated budding of lateral off- 

 shoots. After this process has con- 

 tinued for some time, the chorion pre- 

 sents a uniformly shaggy appearance, 

 owing to the abundant compound vil- 

 losities which cover its surface. 



These villosities, when examined by 

 the microscope, have an exceedingly 

 characteristic appearance. They orig- 

 inate from the chorion by a somewhat 

 narrow stem, and divide into second- 

 ary and tertiary branches of varying 

 size and figure ; some filamentous, 

 others club-shaped, many irregularly 

 swollen at various points. All termi- 

 nate by rounded extremities, giving 

 to the whole tuft a resemblance to 



some varieties of sea-weed. The larger trunks and branches of the 

 villosity contain minute nuclei, imbedded in a nearly homogeneous, or 

 finely granular substratum. The smaller rami- 

 fications appear* under a low magnifying power, 

 simply granular in texture. 



The villi of the chorion are different from 

 any other structure in the body. Whenever 

 any portion of a membrane with villosities of 

 this character is discharged from, or is found 

 in, the cavity of the uterus, it is certain that 

 pregnancy has existed ; for such villosities can 

 only belong to the chorion, and the chorion 

 is a part of the fcetus. The presence of a 

 shaggy chorion is therefore as satisfactory 

 proof of the existence of pregnancy, as if the 

 body of the fetus itself had been found. 



While the villosities just described are in 

 process of formation, the chorion receives a 

 supply of blood-vessels from the body of the 

 embryo. The arteries, which are a continuation of those distributed 

 to the intestine, pass out along the canal of communication to the 

 chorion and ramify over its surface. The embryo at this time has 



I 



COMPOUND VILLOSITY OF THE HUMAN 

 CHORION, ramified extremity. From a 

 three months' fcetus. Magnified 30 

 diameters. 



FlG. 201. 



EXTREMITY OF A VILLOSITY OP 

 THE CHORIOX, magnified 180 

 diameters ; showing the blood- 

 vessels in its interior. 



