904 GENERATION. 



niotic cavity. Bernard, who is cited in the above table as having determined the pres- 

 ence of sugar in the araniotic fluid, has shown that, in animals with a multiple placenta, 

 the amnion has a glycogenic function during the early part of intra-uterine existence. 



With regard to the origin of the arnniotic fluid, it is impossible to say 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 transudation from the vascular structures beneath this 

 membrane. The quantity is apparently too great, especially in the early months, to be 

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

 general surface of the foetus and from the membranes. After the third month, the seba- 

 ceous secretion from the skin of the foetus prevents the absorption of any of the liquid. 



An important property of the amniotic fluid is that of resisting putrefaction and of 

 preserving dead tissues. It is 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 which still communicates freely with the intestine. This is the um- 

 bilical 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 

 mammals, however, the umbilical vesicle is not so important, as nutrition is effected by 

 means of vascular connections between 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 it is compressed between the amnion and the chorion as 

 the former membrane becomes distended. 



When the umbilical vesicle is formed, in the way which we have indicated, it receives 

 two arteries from the two aortae, 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 com- 

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

 obliterated, and it closes at the thirtieth or the thirty -fifth day. The point of communica- 

 tion of the vesicle with the intestine is called the intestinal umbilicus ; and, early in the 

 process of development, 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 discovered at the end of pregnancy. 



The umbilical vesicle presents three coats; an external, smooth membrane, formed 

 of connective tissue, a middle layer of transparent, polybedric cells, and an internal layer 

 of spheroidal cells. The membrane, composed of these layers, encloses a pulpy mass, 

 composed of a liquid containing cells and yellowish granulations. 



Formation of the Allantois and the Permanent Chorion. During the early stages or 

 development of the umbilical vesicle, and while it is being shut off from the intestine, 

 there appears an elevation at the posterior portion of the intestine, which rapidly in- 

 creases in extent, until it forms a membrane of two layers, which is situated between 

 the internal and the external layer of the amnion. This membrane becomes vascular 

 early in the progress of its development, increases in size quite rapidly, and finally com- 

 pletely encloses the internal layer of the amnion and the embryon, the gelatinous mass 

 already described being situated between it and the internal amniotic layer, before this 

 membrane becomes enlarged. While the formation of the two layers of the allantois is 

 quite distinct in certain of the lower orders of animals, in the human subject and in mam- 

 mals, it is not so easily observed ; still there can be no doubt as to the mechanism of its 

 formation, even in the human ovum. Here, however, the allantois soon becomes a single 



