CHAP.XLIII.] UMBILICAL VESICLE. 619 



tern of the chick for its nutrition, throughout the whole period of 

 development within the shell. That portion of the yolk nearest to 

 the vessels becomes quite fluid, and is therefore in a state most fa- 

 vorable for absorption ; it also becomes mixed with the albumen, 

 by which it was originally surrounded, and which enters by en- 

 dosmosis through the yolk membrane ; and it undergoes certain 

 chemical changes, as the experiments of Dr. Prout have shown. 



We have already shown (page 584), that the yolk is in direct 

 continuity with the cavity of the intestine, through the interven- 

 tion of the vitelline duct; and it is therefore possible for the 

 nutrient material to reach the system of the chick by this shorter 

 and more simple course at a very early period of development. 

 Although there can be no doubt of the existence of this tubular 

 communication at one time, it is nevertheless quite certain that 

 throughout the greater part of the period of incubation, the duct 

 is impervious, and the nutrient material of the yoke is absorbed by 

 the vessels ramifying upon the surface of the umbilical vesicle, 

 and is carried by the omphalo-mesenteric vessels to the vascular 

 system of the embryo in the manner above described. 



The yolk sac of the mammalian ovum has a structure very similar 

 to that of the bird. The communication with the intestine is at 

 first very wide, but soon becomes reduced to a narrow tube, the 

 omphalo-mesenteric or vitelline duct. The umbilical vesicle generally 

 disappears at a very early period; and in the embryo calf, not 

 more than six lines in length, according to Bischoff, it is only 

 connected with the embryo by a thread-like pedicle, and is of very 

 small size. In the frog it disappears very early ; while in carnivo- 

 rous animals, and also in the rodents, it remains throughout a con- 

 siderable period of intra-uterine life, and is very highly vascular. 



The vessels of the umbilical vesicle are well shown in an embryo 

 of Dr. Sharpey's. The foetus was 1 - of an inch in length. The 

 vesicle was ^ of an inch in diameter, and the pedicle -^ of an 

 inch long. A beautiful engraving of this embryo will be found in 

 Muller* Physiology, translated by Dr. Baly. 



In the human embryo of from two to three lines in length, 

 Miiller found the duct of the umbilical vesicle very short and wide, 

 and was able to trace its walls in direct continuity with those of 

 the intestine. Dr. Allen Thomson also testifies to the same fact, 

 and Weber has delineated its blood-vessels. It is very distinct in 

 the human embryo about the twentieth day. It lies between the 

 chorion and amnion, and is filled with a yellowish-white yolk. By 

 the third month, it is about four or five lines in diameter ; and 



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