. The development of the chick; an introduction to embryology . (Fig. 1S,3); at all stages of inculia-tion the neck of the allantois forms an open connection Ijetweenthe cloaca atid the allantoic sac. The Umbilicus, ihe closin-e of the body-wall jirogicssivelyi-educes the conununication lietween the emhi-yonic and extra-embryonic body-cavity to a narrow chink between the volk-stalk FROM TWEL^E TO THIRTY-SIX SOMITES 145 and allantoic stalk on the one hand and the attachment of theamnion on the other. The umbilical cord thus consists of anouter tube continuous with the body-wall, enclosing the y
. The development of the chick; an introduction to embryology . (Fig. 1S,3); at all stages of inculia-tion the neck of the allantois forms an open connection Ijetweenthe cloaca atid the allantoic sac. The Umbilicus, ihe closin-e of the body-wall jirogicssivelyi-educes the conununication lietween the emhi-yonic and extra-embryonic body-cavity to a narrow chink between the volk-stalk FROM TWEL^E TO THIRTY-SIX SOMITES 145 and allantoic stalk on the one hand and the attachment of theamnion on the other. The umbilical cord thus consists of anouter tube continuous with the body-wall, enclosing the yolk-stalk and the stalk of the allantois, together with the arteriesand veins of yolk-sac and allantois. It is important to bear inmind that in the region of the neck of the allantois the amnionis attached to the latter at the sides and behind; only the anteriorwall of the allantoic stalk is free (Fig. 82). In other words, thesomatic umbilical stalk is fused with the lateral and caudal wallof the neck of the allantois, a relation that is common to Fig. 81. —Transverse section through the hind-gut and allantois of an em-bryo of 35 s; the section passes through the thirtieth somite. , Allantois. H. G., Hind-gut. L. B., Leg bud. v. M., Ventral mesentery. W. D., Wolffian duct. Other abbreviations as before. Summary of Later History of the Embryonic Membranes. The full history of the eml)i-yonic meml^ranes will l)e given later(Chap. VII), but it seems desirable to give an outline here in orderto avoid repeated recurrence to this subject. The extension ofthe body-cavity in the blastoderm is at first very i-ajiid, but aboutthe fifth day it becomes slow, and the yolk-sac is never com-pletely separated from the chorion. The allantois extends outinto the extra-embryonic body-cavity as a small pear-shapedvesicle by the end of the fourth day. It then enlarges veryrapidly and extends in the form of a flattened sac over and aroundthe em])iyo immediately
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