72. At the lower end of the anal intestines, there projects another vesicle or sac, to which the name of Allantoid has been given. It exists in birds—in all the mammalia—and in the human embryo. In the latter it appears about the fourth week, and by the sixth week it has almost disappeared. Its existence is therefore but short, while the size it attains is always trifling. Not so with the mammalia, in which it acquires great amplitude. The communication of this sac with the bladder, by a canal called the urachus, and which canal is found impervious after the first three or four months of gestation, is admitted by all observers. (Meckel, Baer, Dutrochet.) 73. The umbilical cord or navel string, in man, has been found to spring from a vesicle to which the discoverer has given the name of Erythroid. (Dr. Pockels.) This vesicle had been before observed in the mammalia. (Oken.) The cord appears at the end of the third week. (Pockels.) It then consists of a vein and two arteries, the urachus, a species of gelatine of a ropy nature, called the _gelatine of Wharton_, a portion of the intestinal canal, (larger in proportion as the embryo is younger,) the vesicula umbilicalis, in part, and the omphalo-mesenteric vessels, the whole enveloped by a sheath proceeding from the involucra of the ovum already described. The last three constituent elements of the cord, however, become obliterated or disappear after the third or fourth month of gestation—the others remain permanent throughout that period. 74. The umbilical vein and the arteries are spirally twisted together and in the majority of cases they are so from left to right. (Hunter.) The former contrivance is evidently intended for the purpose of giving the greatest possible extension of tubular passage for the blood compatible with the smallest increase in the length of the cord. The cord which is very short at five or six weeks, becomes at the conclusion of gestation from one foot six to two feet long. (Chaussier.)