4I2 COMPARATIVE ZOOLOGY 



with it forms the body wall; the other bends rapidly 

 downward, carrying the hypoblast with it, and forms 

 the wall of the intestine. The space thus left between 

 the layers of the mesoblast is the body cavity. At the 

 same time, the margin of the germ extends farther and 

 farther over the yolk, till it completely incloses it. So 

 that now we see two cavities a small one, containing 

 the nervous system ; and a larger one below, for the 

 digestive organs. Presently, numerous rows of cor- 

 puscles are seen on the middle layer, which are subse- 

 quently inclosed, forming a network of capillaries, called 

 the vascular area. A dark spot indicates the situation 

 of the heart, which is the first distinctly bounded cavity 



of the circulatory 

 system. It is a 



short tube ^ ing 

 lengthwise just be- 



FIG. 364. Rudimentary Hearts, human: i, venous hind the head, with 

 trunks; a , auricle; 3, ventricle ; 4, bulbus arteripsus. 



causing the blood to flow backward and forward. 

 The tube is gradually bent together, until it forms a 

 double cavity, resembling the heart of a fish. On the 

 fourth day of incubation partitions begin to grow, divid- 

 ing the cavities into the right and left auricles and 

 ventricles. The septum between the auricles is the last 

 to be finished, being closed the moment respiration 

 begins. The blood vessels ramify in all directions 

 over the yolk, absorbing its substance, and all perform- 

 ing the same office ; it is not till the fourth or fifth day 

 that arteries can be distinguished from veins, by being 

 thicker, and by carrying blood only from the heart. 165 



The embryo lies with its face, or ventral surface, 

 toward the yolk, the head and tail curving toward 

 each -other. Around the embryo on all sides the epi- 

 blast and upper layer of the mesoblast rise like a hood 



