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. Beneatli 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 the middle layer, 
4 which are subsequent¬ 
ly enclosed, forming a 
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. 
