VILLI OF THE CHOEION. 



709 



ruptured -^'itti the slightest force, and there is thus caused great difSculty in the 

 examination of the o\Tim. After a few days the external covering of the ovum, 

 which was previously smooth on its siu'face. becomes covered with slight pro- 

 jections, which gradually rise in the form of simple villi, and these, according to 

 Bischoff, have at first the same homogeneous stiiicture as the zona originally pre- 

 sented. But according to Kolliker it may be doubted whether these villi are at 

 first entu-ely homogeneous, and. at all events, he has ascertained that in a veiy 

 early stage of their formation in the human ovum, as in the ovum of from fifteen 

 to eighteen days, described by Coste, and which Kolliker had an opportunity of 

 examining microscopically, the simple villi consist of hollow tubular processes, 

 which are entirely composed of nucleated cells, similar to those of the upper 

 layer of the blastodenn. It is, therefore, most i^robable that according to the 

 view first suggested by Reichert. the villous chorion of the mammal's ovum is a 

 product of the development of the blastodenn, and is formed in fact by the 

 extension of its outer layer, now termed epiblast. 



Fig. 517. 



Fig. 517. — Front and Side Views of an 

 Early Htjjian Ovum Four Tijies the 

 NATURAL SIZE (froiii Keicliert). 



This oviim is sujiposed to be of tliirteen 

 days after impregnation. The surface bare 

 of villi is that next the wall of the uterus, 

 showing at e, the opacity produced by the 

 thickened embryonic disc. The villi covered 

 chiefly the marginal parts of the surface. 



Villi of the Chorion. — A large 

 part of the external surface of the 

 ovum is ill the earlier stage.? beset 

 "with villi, and these villi acquire 



vascularity by the extension into tliem of the blood-vessels of the allan- 

 toid membrane from within. In subsequent stages, however, the form 

 and extent of the development of the villi are subject to great variety in 

 different animals, according to the peculiar form which is assumed" in 

 each tribe by the organic connection established in uterogestation 

 between the uterus and the ovum. 



In the human species the villi appear to become vascular at a very 

 early period, as ascertained by Kolliker in an ovum of between three 

 and four weeks, in which he found that, while a delicate loop of blood- 

 vessels penetrated into each of the villi, the internal part of the villus, 

 which, as before stated, was previously a hollow cellular tube, was now 

 filled with a fibrous connective tissue bearing the simple blood-vessels, 

 and of a structure precisely similar to that of the outer layer of the 

 allantois. It is therefore extremely probable that the primitive zona 

 of homogeneous structure, after being thinned out to great tenuity by 

 the continued expansion of the ovum, disappears entirely, and is 

 replaced by a cellular membranous structure derived from the upper 

 layer of the blastoderm, while the deeper fibro-vascular part proceeds 

 fi'om the outer layer of the allantois ; and from this it necessarily follows 

 that the chorion is no original component of the ovum, but an acquired 

 or newly-formed structure developed from a union of epiblastic and 

 mesoblastic elements. 



Endochorion or Vascular layer of the Allantois. — The separation of 

 the outer vascular layer of the allantois from the deeper layer (hypoblast") 



