. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. 195 the instruments of thouglit, sensation, and motion, or the brain, spinal column, muscles, and skin ; in tlie lower originates the digestive system ; while the middle gives rise to the blood and the oi-£rans of circulation. The next. Fli;. 1(J1. —Vcilical Auctions of au E^'l;, showiu^^ pnigresi-ivc >t:i,uL's of de\elopment: a, primitive streak ; 6, the furrow, becomiug a closed canal iu the last. ]-)liase is the appearance of a faint straight furrow passing through the middle of the
. Comparative zoology, structural and systematic. For use in schools and colleges. Zoology. 195 the instruments of thouglit, sensation, and motion, or the brain, spinal column, muscles, and skin ; in tlie lower originates the digestive system ; while the middle gives rise to the blood and the oi-£rans of circulation. The next. Fli;. 1(J1. —Vcilical Auctions of au E^'l;, showiu^^ pnigresi-ivc >t:i,uL's of de\elopment: a, primitive streak ; 6, the furrow, becomiug a closed canal iu the last. ]-)liase is the appearance of a faint straight furrow passing through the middle of the external layer, called the^rzm- 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 notuchord)—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 tlie middle layer, which are subsequently in- closed, forming a net- work of capillaries, 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 lengthwise just behind the head, with a feeble pulsation, to. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Orton, James, 1830-1877. New York, Harper and brothers
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Keywords: ., bookcentury1800, bookdecade1880, bookpublishe, booksubjectzoology
