THH I'l.ACKXTA. Sf) 



transitory organ, and in the last months of foetal life, 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, adhenng 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 fcetus. 

 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 Pig. 

 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. 



Bitch and Cat. 



In the Bitch and Cat 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. 48, d), extending into the pointed 

 cornua ( ;/') comprised between the amnion, the inner layer of the allan- 

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

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

 very vascular walls. 



The Placenta. 



The placenta varies extremely in different species. In Solipeds it is 

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

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

 chorion {diffused villi or placenta, constituting a chorion frondosum) ; 

 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 verj- red in colour, and consist, like the chorion itself, of 

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

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

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

 nucleated connective tissue, covered by a simple epithelial layer, enclos- 

 ing the capillary vessels, which are arranged in loops made uj) of a 

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

 most, a double, capillary loop. 



The villosities of the fietal placenta, penetrating the newly-formed 

 crypts in the uterine mucous membrane, bring the capillary systems of 

 mother and fcetus into the closest relationship, only the very thin coats 

 of the vessels and the epithelium inter\-ening in the two circulations. 

 There is no fusion, vascular continuity, or direct communication 

 between the maternal and foetal systems, as was at one time taught ; 

 all the important changes that occur taking place through the walls of 

 the capillaries by virtue of osmotic force. 



The function of the placenta, then, is to minister to the nutrition 



