STRUCTURE OF THE VERTEBRATES 69 



ally takes place near the posterior end, and is due to a steady 

 narrowing of the circular opening caused by the growth of the 

 folds toward a common point. When fusion occurs there are two 

 encircling membranes which have developed from the original 

 outer membrane of ectoderm and mesoderm. (1) The inner is 

 the amnion, lined with ectoderm which is continuous with that 

 of the embryonic body, and completely surrounds the embryo 

 except at the body stalk. Fluids diffuse into the amniotic cavity, 

 and the embryo continues its development in a fluid medium. 

 (2) The outer membrane is the chorion and lies next the porous 

 shell. At this stage the embryo is surrounded by the amnion; 

 the yolk sac is ventral; and the chorion surrounds the entire 

 structure. 



The allantois begins its development before the completion of 

 amnion formation. It develops as an outpocketing of the poste- 

 rior end of the gut and grows outward toward the chorion be- 

 tween the amnion and the yolk sac, lying in the extra-embryonic 

 coelom. At this stage the gut is covered by a layer of mesoderm, 

 and the allantoic outpocketing naturally is covered by a meso- 

 dermal layer. The allantois becomes enlarged at the outer end, 

 and as it reaches the chorion (lying against the shell) it spreads 

 until it almost fills the extra-embryonic coelom. 



Blood vessels pass outward from the embryo with the allantois 

 and come into contact with the porous shell. Respiration takes 

 place by osmosis through the thin membranes of the shell into 

 the blood vessels of the chorio-allantoic membrane. The circu- 

 lating blood transports the carbon dioxide from the embryo to 

 the distal capillaries, where waste products are given up and 

 oxygen is received. During embryonic life the aerated blood goes 

 to the embryo through the allantoic veins and is mixed with the 

 unaerated blood of the embryo before reaching the heart. The 

 complete separation of aerated and unaerated bloods does not 

 take place until after the animal hatches and the lungs become 

 functional. 



Later Growth. After the nervous system and the vascular 

 systems are laid down, the organs of excretion begin their devel- 

 opment. The mesoderm gives rise to the muscles, connective tis- 

 sues and bone, and the glands begin their development. It is a 



