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 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. 
Fic. 81. — Transverse section through the hind-gut and allantois of an em- 
bryo of 35s; 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 
