172 EMBRYOLOGY 



The extraembryonic membranes 



In this chapter and in the two following we shall draw heavily upon 

 studies on the chick embryo. To understand adequately the development of 

 the circulatory system it is necessary to know the origin and function of 

 certain extraembryonic membranes. These membranes are lacking in the 

 frog embryo and thus have not yet been considered. 



During the development of the chick embryo four sacs are formed: (l) 

 An amniotic sac surrounds the embryo, protects it from mechanical shock, 

 and provides an aqueous environment for the developing embryo. (2) A 

 chorion forms the outermost layer of cells surrounding the rest of the embryo 

 and embryonic membranes. (3) A yolk sac, which is filled with the yolk of 

 the egg, functions as a digestive organ for the embryo. The cells of the yolk 

 sac secrete digestive enzymes which break down the yolk, and the diffusible 

 products of digestion pass into blood vessels and are carried into the growing 

 embryo. (4) An all an Zoic sac is connected with the hind-gut and has two very 

 important functions. The cavity of the allantois receives the excretory prod- 

 ucts of embryonic development. Urea and uric acid accumulate in it until 

 hatching; then the connection of the allantois with the embryo is broken 

 and the dried up membranes, together with the excretory products, are left 

 in the shell. The second important function is respiration. The blood vessels 

 in the allantois come in close contact with the egg shell, and oxygen diffuses 

 into them from the environment. Similarly carbon dioxide leaves the blood 

 of the allantois and diffuses into the atmosphere. 



These four sacs or membranes are called extraembryonic membranes be- 

 cause they contribute little, if anything, to the structure of the embryo. 

 However, they form an intimate part of the developing embryo, since they 

 supply the embryo with food, mechanical protection, and oxygen and remove 

 from it urea, uric acid, and carbon dioxide. 



The amnion, chorion, yolk sac, and allantois all are composite structures, 

 in the sense that they involve two germ layers. The amnion is composed of a 

 layer of ectoderm plus mesoderm; the chorion is also a double layer of 

 ectoderm and mesoderm; the yolk sac is lined with endoderm and covered 

 with mesoderm; and the allantois is similarly constructed. 



The formation of the amnion begins rather early (Fig. 101). At the head 

 region of the embryo a fold of combined ectoderm and mesoderm begins to 



