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 



