The marked thickening of the first branchial arch indicates 

 the beginning of its conversion into the lower jaw. The 

 vascular area occupies somewhat more than half of the yolk 

 circle by the end of the fourth day. 



During the fifth day (§ 7) , the separation of the 

 embryo reaches the greatest degree, and the urinary sac 

 becomes the organ of respiration. The intestinal umbilicus 

 becomes narrow and is transferred into the yolk duct. The 

 anterior and posterior entrances into the digestive canal 

 join together so that the gutter-shaped part of the intestine 

 disappears. The urinary sac, very rich in blood vessels and 

 on the right side of the embryo between the amnion and the 

 serous membrane lying above it, extends to the surface of the 

 yolk in the form of the serous layer of the ectoderm. At 

 this point, the embryo is so distorted that the head and tail 

 come in contact with each other. The body cavity still forms 

 somewhat at the neck, but. the liver is already placed in the 

 body. 



One can easily distinguish the esophagus, stomach and 

 intestines, and the respiratory tube from the esophagus. In 

 the region of the developing pancreas, the primary intestinal 

 loop corresponding to the duodenum forms. The caecum and 

 large intestines are still poorly developed, but the posterior 

 passage opening is already present in the form of an umbilical 

 slit. The Wolff's bodies are strongly enlarged and extremely 

 rich in blood. On their internal surface, the rudiment of the 

 sexual glands appears as a longitudinal strip. 



In the heart, at the "central venous sac" (auricle), the 

 beginning of the extension is marked. A partition divides 

 the "heart chamber" (ventricle) into two parts connected by 

 an elongated opening. In the stem of the aorta are two 

 orifices. 



The extremities lose their leaf-shaped form and become 

 chisel -shaped. In the shoulder and thigh, dark patches form 

 the rudiments of skeletal elements; correspondingly, in the 

 forearm and shank two dark strips appear. The upper jaw has 



327 



