Embryologic Development of Animals 455 



digestive tract, including the liver and pancreas; the respiratory tract, 

 including the lungs, trachea, and larynx; the bladder; the prostate; the 

 urethra; the yolk sac; the allantois. 



The detailed description of the embryologic origin of each tissue 

 and organ cannot be given, but a few typical examples will be suffi- 

 cient. As the embryo develops, the upper part of the yolk sac forms the 

 tubular primitive m,id- and hindgut with its outgrowth, the liver, when 

 the embryo is about three weeks old. The saclike "hearf begins to beat 

 soon after this time. By the third week the yolk sac has numerous 



Yo/k Sac 



Muscu/ar Layer- 

 of Uterus 



Basal Plate- 

 of Placenta 



Umbilical Coraf- 



Amnion 



Chorion & Oecidua 

 Copsularis 



Fig. 226. — Human fetus shown in normal position in a section of the uterus. 

 The chorion is the outer embryonic membrane and the amnion is the inner. (From 

 Potter: Textbook of Zoology, The C. V. Mosby Co.; modified after Ahlfeld.) 



' blood islands" for developing the embryonic vitelline circulation. The 

 embryonic disk infolds (invaginates) to form a troughlike groove (neural 

 groove) , the open, upper side of which later closes to form a hollow tube 

 (neural tube). From the anterior end of the tube will develop the vari- 

 ous parts of the brain and cranial nerves, while the remainder forms the 

 spinal cord with its spijial nerves. 



Before five weeks the embryo externally shows a head with rudimen- 

 tary eyes, an external tail, and a neck with four pairs of gill arches and 



