482 Description of the Membranes of 



bilical vessels, I did not hesitate to conclude that an ailantois 

 was developed at a more advanced stage of the developement 

 of the embryo. 



I took occasion to observe, that, as the growth of the foetus 

 advanced, the circulating fluids became necessarily more 

 charged with decomposed particles of the organised substance ; 

 and that, although the extended surface of minutely subdi- 

 vided blood-vessels afforded by the vitelline sac might serve 

 both for respiration and nutrition at the earliest stages of de- 

 velopement, yet that, a late period, an accessory apparatus to 

 that end appeared to be necessary, as the embryo acquired 

 additional bulk and parts. In fact, in all the Reptilia in which 

 the respiratory function of the foetus is not performed by the 

 extension of vascular filaments from the sides of the neck, 

 an ailantois, or ccecal process, organised by umbilical or hy- 

 pogastric vessels, is produced from the terminal portion of 

 the intestinal tube. The period of the developement of this 

 accessory respiratory organ, in birds, is immediately subse- 

 quent to the budding forth of the locomotive extremities. In 

 the placental Mammalia, where the vitelline sac and vitellus 

 are relatively smaller, the ailantois makes its appearance 

 much earlier, but is developed in different proportions in the 

 different orders. It is subservient, in all the placental Mam- 

 malia, to one important function, viz. the transference to the 

 exterior enveloping membrane, or chorion, of the hypogastric 

 or umbilical arteries ; and in these Mammalia the umbilical 

 vessels coextended with the allantoic ccecum seek a more 

 intimate contact with the vascular surface of the womb, and 

 proceed to organise the chorion, shooting out into villi, either 

 extended over the whole surface, as in the mare ; or disposed 

 in circumscribed tufts, as in Ruminantia ; or limited to one 

 place, and forming a single placenta, as in the human subject, 

 and in all uuguiculate Mammalia.* 



In the bird and reptile, the umbilical vessels are limited to the 

 ailantois, and do not extend beyond that membrane to the cho- 

 rion ; ailantois, therefore, in these plays a primary part in the re- 

 spiration of the foetus. In the placental Mammalia, on the other 

 hand, its office as a temporary respiratory organ is secondary, 

 but it is essential as a means of transference of the umbilical 

 vessels to the chorion ; it therefore preexists to the placenta, 

 and without it the placenta could not be formed; for, if we 

 consider that the embryo is formed within the bag of the 

 chorion, and originally free from any connexion with that 

 membrane, there must of necessity be some support for the 



* Pub. Phil. Trans., 183+, p. 342. 



