. The biology of the amphibia. Amphibians. THE NERVOUS SYSTEM 357 aggregation of special clusters of cells to discharge in fiber paths having specific functions. The brain arises during development by a differentiation of the anterior part of the neural plate, the edges of which roll over during gastrulation to form a tube. Very early three enlarge- ments common to the brains of all vertebrates develop. These are the forebrain, midbrain and hindbrain. The forebrain again becomes differentiated into a telencephalon and a thalamen- cephalon, while the hindbrain develops a cerebellum poorly marke
. The biology of the amphibia. Amphibians. THE NERVOUS SYSTEM 357 aggregation of special clusters of cells to discharge in fiber paths having specific functions. The brain arises during development by a differentiation of the anterior part of the neural plate, the edges of which roll over during gastrulation to form a tube. Very early three enlarge- ments common to the brains of all vertebrates develop. These are the forebrain, midbrain and hindbrain. The forebrain again becomes differentiated into a telencephalon and a thalamen- cephalon, while the hindbrain develops a cerebellum poorly marked off from the remainder of the hindbrain, the medulla oblongata. Thickenings or other differentiations appear in all five regions. The original cavity of the tube remains to form the ventricles of the A B C Fig. 125.—Diagrammatic cross-section of the forebrains of (A) salamander, (B) frog, and ((7) caecilian, to show the principal nuclei. B., nucleus basalis; , nucleus basimedialis superior; Co., cortex olfactoria; D., area dorsalis pallii; L., area lateralis pallii; M., area medialis pallii. (After Kuhlenbeck.) Forebrain.—The forebrain of Salientia is shorter and more compressed than that of most urodeles. Nevertheless, the evagination of the hemispheres is carried farther, since the unpaired ventricle at the posterior end is smaller in frogs than in urodeles. Two enlargements at the anterior end of the hemi- spheres are the olfactory lobes from which the olfactory nerves arise. They are ventral in Hynobius as in frogs, but in the latter they are fused in the midline. The ventral position of the lobes is probably primitive, although in most urodeles they have a more lateral position, markedly so in the newt and in Siren. In the specialized Eurycea the lobes are also ventral (Rothig. 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 no
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Keywords: ., bookcentury1900, bookpublishernewyorkmcgr, booksubjectamphibians
