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EMBRYOLOGY. 125 
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 sub- 
divides 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 undevel- 
oped 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 rudi- 
mentary condition of the lungs in the human being, or in the 
embryo ofthe higher animal, as in these the lungs are deve 
oped as buds from the alimentary canal, the pedicle by whic 
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 
orless 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, 
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