FORMATION OF THE UMBILICAL VESICLE. 367 



stated by Robin, to be the best fluid for the preservation of 

 the embryonic tissues, when it is desired to keep them for 

 examination. 



Formation of the Umbilical Vesicle. As the visceral 

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

 of the embryon, that portion of the blastoderm from which 

 the intestinal canal is developed presents a vesicle, which is 

 cut off, as it were, from the abdominal cavity, but still com- 

 municates freely with the intestine. This is the umbilical 

 vesicle. On its surface, is a rich plexus of blood-vessels ; and 

 this is a very important organ in birds and in many of the 

 lower orders of animals. In the human subject and in mam- 

 mals, however, the umbilical vesicle is not so important, as 

 nutrition is effected by means of vascular connections be- 

 tween the chorion and the uterus. The vesicle becomes 

 gradually removed farther and farther from the embryon, as 

 development advances, by the elongation of its pedicle, and 

 is compressed between the amnion and the chorion as the 

 former membrane becomes distended. 



When the vesicle is formed, in the way which we have 

 indicated, it receives two arteries from the two aortse, and 

 the blood is returned to the embryon by two veins, which 

 open into the vestibule of the heart. These are called the 

 omphalo-mesenteric vessels. At about the fortieth day, one 

 artery and one vein disappear, and, soon after, all vascular 

 connection with the embryon is abolished. At first there is 

 a canal of communication with the intestine, called the om- 

 phalo-mesenteric canal. This is gradually obliterated, and 

 closes at the thirtieth or 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 develop- 

 ment, there is here a true hernia of a loop of intestine. The 

 umbilical vesicle remains as a tolerably prominent structure 

 as late as the fourth or fifth month, but it may often be dis- 

 covered at the end of pregnancy. 

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