DEVELOPMENT OF ACCESSORY STRUCTURES. 157 



extends around the neural canal and the chorda dorsalis, and forms the 

 arches and bodies of the vertebrae. They become divided transversely into 

 four-sided segments. 



The Mesoblast now separates into two layers ; the external, joining with 

 the epiblast, forms the somatople ure ; the internal, joining with the hypo - 

 blast, forms the splanchnopleure ; the space between them constituting the 

 pleuro-peritoneal cavity. 



Visceral Laminae. The walls of the pleuro-peritoneal cavity are 

 formed by a downward prolongation of the somatopleure (the visceral 

 lamina], which, as they extend around in front, pinch off a portion of the 

 yelk sac (formed by the splanchnopleure), which becomes the primitive 

 alimentary canal ; the lower portion, remaining outside of the body cavity, 

 forms the umbilical vesicle, which after a time disappears. 



Formation of Foetal Membranes. The Amnion appears shortly 

 after the embryo begins to develop, and is formed by folds of the epiblast 

 and external layer of the mesoblast, rising up in front and behind, and on 

 each side; these amniotic folds gradually extend over the back of the 

 embryo to a certain point, where they coalesce, and enclose a cavity, the 

 amniotic cavity. The membranous partition between the folds disappears, 

 and the outer layer recedes and becomes blended with the vitelline mem- 

 brane, constituting the chorion, the external covering of the embryo. 



The Allan tois. As the amnion develops, there grows out from the 

 posterior portion of the alimentary canal a pouch, or diverticulum, the 

 allantois, which carries bloodvessels derived from the intestinal circulation. 

 As it gradually enlarges, it becomes more vascular, and inserts itself be- 

 tween the two layers of the amnion, coming into intimate contact with the 

 external layer. Finally, from increased growth, it completely surrounds 

 the embryo, and its edges become fused together. 



In the bird, the allantois is a respiratory organ, absorbing oxygen and 

 exhaling carbonic acid ; it also absorbs nutritious matter from the interior 

 of the egg. 



Amniotic Fluid. The amnion, when first formed, is in close contact 

 with the surface of the ovum ; but it soon enlarges, and becomes filled 

 with a clear, transparent fluid, containing albumen, glucose, fatty matters, 

 urea and inorganic salts. It increases in amount up to the latter period of 

 gestation, when it amounts to about two pints. In the space between the 

 amnion and allantois is a gelatinous material, which is encroached upon, 

 and finally disappears as the amnion and allantois come in contact, at about 

 the fifth month. 



