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THE CHOEIOX 707 



membrane, which acquires more or less of a composite villous structure, 

 becomes vascular throughout the whole or a part of its extent, and 

 which, by its farther development, comes to form the principal means on 

 the side of tlie ovum of establishing an organic connection between the 

 embryo and the uterus. While the name of prochormi, or primitive 

 chorion, might without impropriety be given to the altered and expanded 

 zona pellucida as the sole early covering of the ovum in mammals, the 

 term chorion is most suitably reserved for the newly formed membrane 

 here referred to. 



By some aiithors. indeed, the name of chorion has been applied to the external 

 covering of the ovum of all animals without regard to its source or its relations 

 to other parts. Thus by some the vitelline membrane has been regarded as 

 a chorion when it appeared that no other membrane existed external to it ; and 

 by others the name has been given to such adventitious parts as the albumen, 

 shell, or shell membrane of the ovipara : but such a use of the term chorion is 

 liable to create confusion, and it seems more expedient that it should be restricted 

 to the peculiar external covering of the mammiferous ovum, which, as will be 

 shown hereafter, is not an original constituent of the ovum like the vitelline 

 membrane, but a structure of new formation in the course of development. 



Fig. 515. —View op the Chorion of the Human Ovum of 

 ABOUT Four or Five Weeks, opened (from KoUiker after 

 Allen Thomson). Natural size. 



This figure gives a general view of the villous structure of 

 the chorion previous to the formation of a placenta, and shows 

 the large space which frequently intervenes at an early period 

 between the amnion and chorion. 



At a very early period in the majority of mam- 

 mals, and especially in the human species, tlie 

 chorion acquires numerous villous processes over 

 the whole or a part of its outer surface. These 

 soon undergo a great development, and constitute a peculiar feature 

 in the human ovum, whence the membrane has been known in'human 

 embryology as the chorion frondosum, or shaggy chorion. 



Tlie blood-vessels borne by the developed villi of the chorion, and named 

 umbilical m human anatomy, are originally derived from those of the 

 allantoid membrane, and are the seat of an extended circulation of the 

 foetal blood in a system of outgoing arteries and returning veins witli their 

 intervening widely diffused capillary vessels. It is by this system of vas- 

 cular chorionic villi being brought into contact or close proximity with the 

 blood-vessels of the uterus, that the essential conditions of nterogestation, 

 as regards the continued supply of nourishment to the foetus and the 

 aeration of its blood, are secured in the whole class of mammiferous 

 animals. There is, however, very great difference among these animals 

 in the extent and form of the development of the villous structure of 

 the chorion now referred to, as well as of the concomitant changes which 

 occur in the uterus itself, by which a more or less intimate organic 

 nnion is established between the maternal parent and the offspring. The 

 history of these differences belongs to the account of the structure and 

 formation of the placenta, which will be given hereafter. At this place 

 it will be sufficient to state that, while in some animals, as the pachy- 

 dermata and cetacea, the connection between the ovum and uterus 



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