200 COMPARATIVE ZOOLOGY. 



tube of epiblast surrounded by mesoblast, which is itself 

 covered by epiblast. This tube becomes the brain and 

 spinal cord, whose central canal, enlarging into the ven- 

 tricles -of the brain, tells the story of its original forma- 

 tion. Beneath the furrow, a delicate cartilaginous thread 

 appears (called notochord) the predecessor of the back- 

 bone. Meanwhile the mesoblast has divided into two 

 layers, except in the middle of the animal, beneath the 

 spinal cord, and in the head. One of these layers remains 

 attached to the epiblast, and 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 ex- 

 tends farther and farther over the yolk, till it completely 

 encloses it. So that now we see two cavities a small 

 one, containing the nervous system ; and a larger one be- 

 low, for the digestive organs. Presently, numerous rows 



of corpuscles are seen 



on ^ ie m iddle ^ a J er ? 

 which are snbsequent- 

 ly enclosed, forming a 

 __. _ , . net-work of capillaries, 



FIG. 168. Rudimentary Hearts, human : 1, venous 



trunks ; 2, auricle ; 3, ventricle ; 4, bulbus arte- Called the Vascular area. 

 riosus. , , . .. 



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 lying lengthwise just behind the head, with a feeble 

 pulsation, causing the blood to flow backward and for- 

 ward. 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 ven- 

 tricles. The septum between the auricles is the last to 

 be finished, being closed the moment respiration begins. 



m' 



'^HB 



