Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/183

Rh 40),—the fish without skull or brain. In all other Vertebrata, however, the anterior part of the spinal marrow, in the course of development, expands into a vesicle, which subdivides into three; the anterior of these three vesicles divides into two, and the posterior into two, the middle remains undivided; thus five vesicles (Fig. 177, 1, 2, 3, 4, 5) are formed out of the swelling of the anterior portion of the spinal marrow. These vesicles are called, translating their German names literally, the Fore brain, Between brain. Middle brain, Hind brain, and Hindmost brain, the different parts of the brain being developed from them. The brain of adult man, although highly complex in its organization, is nevertheless represented, at an early period of life, by five vesicles, being undistinguishable from those of an embryo dog, rabbit, bird, or fish. In fishes like the Myxine and Lamprey the brain remains in this undeveloped condition, thus exhibiting permanently the stages of the brain that are transitory in the higher animals. Every one knows that in breathing the air passes through the windpipe to the lungs, and that the food goes to the stomach through a separate and distinct tube. If, however, a Garpike be examined, its lung-like air-bladder is seen to communicate with the alimentary canal by a tube, the air-duct. This arrangement represents perfectly the rudimentary condition of the lungs in the human being, or in the embryo of the higher animal, as in these the lungs are developed as buds from the alimentary canal, the pedicle by which they are attached to it becoming later the windpipe, which corresponds to the air-duct of the Gar. The organs of Respiration naturally suggest those of Circulation. The successive stages through which the heart and blood-vessels of mammals pass in the course of development are more or less well represented by the vascular apparatus of the fish, batrachian, reptile, and bird. The termination of the Digestive, Reproductive, and Urinary apparatus in a Cloaca,