8o OBSTETRICAL PHYSIOLOGY. 



with the intestinal canal of the foetus ; being in reality the vitellus 

 surrounded by the blastoderm upon which the embryo is first formed ; 

 and it bears a perfect analogy to the yolk of the &gg, except that it is not 

 ultimately enclosed within the foetal abdomen, It is a transitory organ, 

 and in the last months of foetal, though in solipeds traces of it continue 

 until parturition, it is always more or less atrophied, its cavity has 

 disappeared, and nothing is left of it but a small reddish-brown cord, 

 adhering to one of the sides of the infundibulum. Its vessels also atrophy 

 in the same manner, the artery being nearly always found reduced 

 to the dimensions of a thread. 



Its use is evidently to contain nutriment for the foetus, before the 

 development of the placenta ; though it may also serve other purposes. 

 It is the first organ which elaborates and supplies blood to the foetus. 

 In some instances the chorion has been found perforated at its junction 

 with the umbilical vesicle, which was therefore in communication with 

 the cavity of the uterus. 



Differences. 



Ruminants and Fig. 



In Ruminants and the Pig, the umbilical vesicle is longer than in 

 solipeds ) it also bulges in the middle, and its ends terminate in a canal. 

 It is longest about the twenty-fifth day, and disappears very early ; no 

 traces of it can be observed between the second and third month, after 

 the abdominal parietes have been formed. 



BiUh and Cat. 



In the Bitch and Cat, however, it remains very developed up to the 

 time of parturition, and in form resembles the allantois of the Pig. It 

 is a transversely elongated sac (Fig. 38, ^), extending into the pointed 

 cornua (^) comprised between the amnion, the inner layer of the allan- 

 tois (h), and the placenta {U) ; it is provided at its middle part with a 

 narrow pedicle (^), which is prolonged into the umbilical cord and has 

 very vascular walls. 



The Placenta. 



The //d;^/?^/^ varies extremely in different species. In solipeds it is 

 constituted by a multitude of short villosities or filiform papillae, which 

 are spread in a uniform manner over the whole external surface of the 

 chorion {diffused villi or placenta, constituting a chorion fro?idosum) ; 

 though there is sometimes observed a tendency to bare patches, one 

 especially being noticed opposite the os uteri, where there is no mucous 

 membrane for the villi to penetrate. These villi are received into cor- 

 responding depressions or follicles in the lining membrane of the uterus. 

 The villi are very red in color, and consist, like the chorion itself, of an 

 epithelial and a vascular layer, they being, in fact, the terminal rami- 

 fications of the vessels of the umbilical cord. They are slender and 

 easily torn ; and each is composed of a small quantity of delicate nu- 

 cleated connective tissue, covered by a simple epithelial layer, enclosing 

 the capillary vessels, which are arranged in loops made up of a principal . 

 arteriole and two veins, there being generally only a single, or at most, 

 a double, capillary loop. 



The villosities of the foetal placenta, penetrating the newly formed 



