710 rXEEOGESTATION. 



M'liicli contrans the fluid, is sufficiently oljvious in many animals, as. for 

 example, in the sheep or pig. But in the human subject, assuming that the 

 vascular elements of the chorion are derived from the allantois as in animals, 

 which there is no reason to doubt, it has been found difficult to determine 

 the exact manner in which they first pass into the villi, in consequence of the 

 very early time and extreme rapidity of the development of the allantois. 

 But notwithstanding the observations previously mentioned of a nonvascula,r 

 liediculated vesicle in relation with the allantois, passing from the umbilicus 

 of the embryo into the space between the amnion and chorion : yet, in the 

 great majority of instances, so rapid is the expansion of the membrane, that 

 even in ova of from three to four weeks old it has been found impossible 

 to trace more than the connection of the pedicle of the allantois through the 

 urachus with the genito-urinary sinus ; and in all the cases which have been 

 observed, already the umbilical vessels are found detached from the deeper mem- 

 brane, and passing ■\\'idely over the whole interior of the chorion to penetrate 

 everj'where into its villi. We are led thus to su^Dpose that by the early and rapid 

 expansion of the outer layer, or by some other naode of development of its 

 fibrous and vascular elements, the blood-vessels of the human allantois have been 

 brought into combination with the cellular laj^er of the chorion, and have pene- 

 trated everywhere into its villi, into the whole of which blood-vessels and fibrous 

 elements may at first be traced. According to this view it is to be understood 

 that while the vascular layer of the allantois may thus become widely diffused, 

 the vesicular or deeper layer may have only a comparatively restricted range of 

 development. 



UTEROGESTATION : PLACENTATION. 



Incapsulation of the Ovum in the Becidua. — The further history of 

 the chorion may be best given along with that of the structures by which 

 the ovum is fixed in the uterus, and organic union established between 

 it and the maternal system. This union is efFected by the close inter- 

 penetration of the vascular villi covering the surface of the chorion 

 with a soft and spongy layer of substance, Avhicli is the product of a 

 rapid enlargement or a sort of hypertrophy of the lining membrane of 

 the uterus. To the latter substance the names of decidua and cadwa 

 are given, from the circumstance that it is separated from the uterus 

 at birth along with the foetus and its membranes. Not only is the 

 ovum from an early period completely imbedded in a covering of 

 decidua, but there takes place at a somewhat later period of uteroges- 

 tation, in a limited area of one side, a greater enlargement of the 

 vascular chorionic villi, and in close combination with, and surround- 

 ing these villi, a corresponding increased development of the decidual 

 substance, by which there is produced the large discoid mass of 

 complex structure, named the placenta (or uterine cake), through 

 which the nourishment of the foetus and the ai'ration of its blood arc 

 mainly carried on during the latter three-fourths of the period of 

 uterogestation. 



This placenta continues to increase in size with the foetus and its 

 membranes, and as pregnancy advances, considerable changes take 

 place in the relations of some of these parts, for the fuller comprehen- 

 sion of which it will be necessary to state the successive steps by 

 which the ovum becomes fixed in the uterine decidua, or mcajisulaied, 

 and the manner in which the development of the chorion, decidua, and 

 placenta proceeds. 



Earliest Observed Htinian Ova. — In two distinct cases of younj^ 



