Early in intrauterine life, the allantois is given off from the posterior end of the primitive intestine; its walls are composed of entoderm and mesoderm provided by the hind end of the primitive groove posterior to the anal membrane. The anal membrane consists of ectoderm and entoderm, i. e., it has no mesodermic tissue. As the anal membrane becomes curled forward to form in conjunction with the lateral infoldings of the embryo the hypogastrium and to contract the umbilical opening, the junction of the intra-embryonal and extra-embryonal portions of the allantois is contracted. A spur of tissue exists between the allantois and the yolk sac or umbilical vesicle. This spur increases and grows backwards through the primitive intestine until it divides the primitive gut into an upper part (intestine) and a lower part, continuous with the allantois, which forms the urinary bladder. Thus the walls of the bladder are formed,
BINNIE JF. DEVELOPMENT OF THE URACHUS. JAMA. 1906;XLVII(2):109–110. doi:10.1001/jama.1906.25210020029002h
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