THE EXTRAEMBRYONIC MEMBRANES 



173 



HEAD FOLD OF AMNION 



AORTIC ARCH 



VITELLINE ARTERY—' 



AREA VASCULOSA 



Fig. 101. The head fold of the amnion and the circulation of the blood at 

 about 38 hours of incubation time. A crest of combined ectoderm and mesoderm 

 rises over the head region and subsequently moves over the dorsal surface of the 

 embryo. The fold is the head fold of the amnion. 



The circulation of the blood is indicated by arrows. In the area vasculosa the 

 vitelline arteries branch into finer and finer vessels which unite to form the 

 vitelline veins. Food from the yolk enters the circulation in the area vasculosa. 



move back over the surface of the embryo. This comprises the head fold of 

 the amnion. Somewhat later a similar fold appears posterior to the embryo — 

 the tail fold of the amnion (Fig. 102). These folds approximate each other 

 and finally fuse. While the head fold and tail fold are developing, lateral 

 folds of the amnion appear (Fig. 103). These extend both above and below 

 the embryo, with the result that the embryo is completely enveloped in a sac, 

 the amnion. The chorion, which is the outermost layer of the folds, becomes 

 detached from the amnion except for one very localized region. The com- 

 bined layers of mesoderm and ectoderm are sometimes designated as 

 somatopleure. The mesoderm of the amnion forms muscle cells which con- 

 tract rhythmically, and thus the embryo is rocked back and forth within the 

 fluid of the amniotic cavity. 



Details of the development of the yolk sac will be given in Chapter 13. 

 At this time it is necessary only to understand that the endoderm, accom- 

 panied by mesoderm containing blood vessels, spreads over the yolk of the 

 egg and finally encloses it in a yolk sac. This yolk sac communicates with 

 the embryo by way of the yolk stalk (Fig. 103) . Vitelline arteries carry blood 

 to the yolk sac and vitelline veins drain it (Fig. 102). 



For the present the allantois will be considered as an endodermal out- 



