DEVELOPMENT. 195 
the instruments of thought, sensation, and motion, or the 
brain, spinal column, muscles, and skin; in the lower 
originates the digestive system; while the middle gives 
rise to the blood and the organs of circulation. The next 
Fic. 161.—Vertical Sections of an Egg, showing progressive stages of development: 
f—T— 3) 
a, primitive streak ; b, the furrow, becoming a closed canal in the last. 
phase is the appearance of a faint straight furrow passing 
through the middle of the external layer, called the prim- 
itive stripe, which corresponds to the axis of the future 
body.” The walls of the furrow gradually rise, and at 
last meet, forming a canal, larger at one end than the 
other, which is filled with a fluid—the beginning of the 
brain and spinal marrow. Beneath the furrow, a delicate 
cartilaginous thread appears (called notochord)—the rep- 
resentative of the backbone. At the same time, the mar- 
gin 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. Pres- 
ently, numerous rows 
of dark-yellow corpus- 
cles are seen on the » 
middle layer, which 
a subsequently in- Fia. 162.—Rudimentary Hearts, human: 1, venous 
closed, forming a net- trunks; 2, auricle; 3, ventricle; 4, bulbus ar- 
work of capillaries, a 
called the vascular area. A dark spot indicates the situ- 
ation of the heart, which is the first distinctly bounded 
cavity of the circulatory system. It is a short tube lying 
iengthwise just behind the head, with a feeble pulsation, 
