above it. At the thirteenth day, the ends of the tubules 

 become welded together, while the surface of the lungs 

 becomes smooth again. 



Further differentiation of the vascular system leads to 



the anterior part of the body, which becomes supplied with 



arterial blood from the left ventricle; both ventricles supply 

 the posterior part. 



In the brain, the four-hillock body has the shape of 

 two widely separated follicles. Anterior to them lies the 

 big brain, and the cerebellum is situated posteriorly; as a 

 result, the whole brain, according to Baer, resembles the 

 outlines of the ace of clubs. 



The iris of the eye begins pigmentation from the side of 

 its pupil border. 



The auditory canal is situated in the open furrow of the 

 wedge-shaped bone and is covered in obliquely located drum 

 membranes . 



During the fourth stage, from the fourteenth through the 

 sixteenth day of incubation (§ 12) , the urinary sac covers 

 practically the entire egg contents: the greatly wrinkled 

 yolk sac, the embryo and most of the albumen. It represents 

 an intact membrane adjoining the shell membrane and carries the 

 name "chorion." 



Since it will soon move out of the shell, the embryo under- 

 goes active respiratory movements. Some intestinal loops hang 

 from the umbilicus, which soon begin to be gradually drawn 

 backwards into the abdominal cavity. The feather rudiments 

 elongate but still do not appear clearly. 



The organs of respiration, the kidneys, the central nervous 

 system and the eyes continue developing, not undergoing remarkable 

 quantitative changes. Differences in the sexual apparatus become 

 more distinct. At the entrance to the nostrils, the scales 

 characteristic of the chicken family appear. 



335 



