8o6 EMBRYOLOGY 



how much of it is derived from the general surface of the foetus, how 

 much from the urine, and how much from the amnion itself, by transu- 

 dation from the vascular structures beneath this membrane. The quan- 

 tity apparently is too great, especially in the early months, to be derived 

 entirely from the urine of the foetus ; and there probably is an exudation 

 from the general surface of the foetus and from the membranes. After 

 the third month the sebaceous secretion from the skin of the foetus pre- 

 vents the absorption of any of the liquid. An important property of 

 the amniotic liquid is that of resisting putrefaction and of preserving 

 dead tissues. 



Formation of the Umbilical Vesicle (Yolk-sac}. As the visceral 

 plates, which will be described hereafter, close over the front of the 

 embryo, that portion of the blastoderm from which the intestinal canal 

 is developed presents a vesicle, which is cut off from the abdominal 

 cavity but which still communicates freely with the intestine. This is 

 the umbilical vesicle. On its surface is a rich plexus of bloodvessels ; 

 and this is an important organ in birds and in many of the lower forms 

 of animals. In the human subject and in mammals, however, the um- 

 bilical vesicle is not so important, as nutrition is secured by means of 

 vascular connections between the chorion and the uterus. The vesicle 

 becomes gradually removed farther and farther from the embryo, as 

 development advances, by the elongation of its pedicle ; and it is com- 

 pressed between the amnion and the chorion as the former membrane 

 becomes distended. 



When the umbilical vesicle is formed, it receives two arteries from 

 the two aortae, and the blood is returned to the embryo by two veins 

 which open into the vestibule of the heart. These are called the om- 

 phalo-mesenteric vessels. At about the fortieth day one artery and 

 one vein disappear, and soon after, all vascular connection with the 

 embryo is lost. At first there is a canal of communication with the 

 intestine, called the omphalo-mesenteric canal. This is gradually obliter- 

 ated, and it closes between the thirtieth and the thirty-fifth day. The 

 point of communication of the vesicle with the intestine is called the 

 intestinal umbilicus ; and early in the process of development, there is 

 here a hernia of a loop of intestine. The umbilical vesicle remains as a 

 rather prominent structure as late as the fourth or fifth month, but it may 

 be discovered at the end of pregnancy. 



The umbilical vesicle presents three coats : an external smooth mem- 

 brane, formed of connective tissue, a middle layer of transparent poly- 

 hedric cells, and an internal layer of spheroidal cells. The membrane, 

 composed of these layers, encloses a pulpy mass, composed of a liquid 

 often containing cells and yellowish granules. 



