Archive image from page 164 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 FROM TWELVE TO THIRTY-SIX SOMITES 145 and allantoic stalk on the one hand and the attachment of the amnion on the other. The umbilical cord thus consists of an outer tube (somatic stalk) continuous with the body-wall, enclosing the yolk-stalk and the stalk of the allantois, together with the arteries and veins of yolk-sac and allantois. It is important to bear in mind that in the region of the neck of the allantois the amnion is attached
Archive image from page 164 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 FROM TWELVE TO THIRTY-SIX SOMITES 145 and allantoic stalk on the one hand and the attachment of the amnion on the other. The umbilical cord thus consists of an outer tube (somatic stalk) continuous with the body-wall, enclosing the yolk-stalk and the stalk of the allantois, together with the arteries and veins of yolk-sac and allantois. It is important to bear in mind that in the region of the neck of the allantois the amnion is attached to the latter at the sides and behind; only the anterior wall of the allantoic stalk is free (Fig. 82). In other words, the somatic umbilical stalk is fused with the lateral and caudal wall of the neck of the allantois, a relation that is common to all amniota. Fig. 81. —Transverse section through the hind-gut and allantois of an em- bryo of 35 s; the section passes through the thirtieth somite. Details diagrammatic. All., 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 embryonic membranes will be given later (Chap. VII), but it seems desirable to give an outline here in order to avoid repeated recurrence to this subject. The extension of the body-cavity in the blastoderm is at first very rapid, but about the fifth day it becomes slow, and the yolk-sac is never com- pletely separated from the chorion. The allantois extends out into the extra-embryonic body-cavity as a small pear-shaped vesicle by the end of the fourth day. It then enlarges very rapidly and extends in the form of a flattened sac over and around the embryo immediately beneath the chorion with which it forms
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