THE FROG AND VERTEBRATES IN GENERAL 209 



I. CENTRAL NERVOUS SYSTEM 



The central nervous system arises early in the development of 

 the embryo by the definite infolding of a portion of the ectoderm 

 of the dorsal body wall to form a hollow neural ture which runs 

 practically the entire length of the body. When first formed the 

 neural tube shows very little differentiation, but in a short time 

 the anterior end becomes enlarged and modified to form three 

 divisions, known as the fore-rrain, mid-rrain, and hind-brain. 

 From these the entire brain of the adult Frog is formed. The 

 remainder of the neural tube posterior to the brain retains through- 

 out life a considerable degree of its early character and becomes the 

 spinal cord of the adult. ( W. fs. 140 ; 174, K.) 



A. The Brain. Consideration may now be given to the external 

 structure of the fully-developed Frog's brain as seen from the 

 dorsal surface. The extreme anterior end consists of the fused 

 olfactory lores. A pair of olfactory nerves which innervate 

 the olfactory sense organs can be traced anteriorly from this re- 

 gion. Posteriorly, the olfactory lobes merge into a pair of elon- 

 gated bodies, the cererral hemispheres, which together con- 

 stitute the cererrum. Posterior to the cerebrum is an unpaired 

 structure, known as the diencephalon, which bears the pineal 

 rody. The parts of the brain so far enumerated develop from the 

 fore-brain. Just back of the diencephalon is a pair of optic 

 lores which constitute the part of the mid-brain seen from the 

 dorsal surface. Lying posterior and in close proximity to the 

 optic lobes is a transverse ridge of tissue which constitutes the 

 cererellum. Just back of the cerebellum is the medulla or- 

 longata which appears as a somewhat enlarged portion of the 

 anterior end of the spinal cord. The latter continues posteriorly 

 through the body without further marked differentiation. The 

 cerebellum and the medulla develop from the hind-brain. (W. f. 

 141, B.) 



When viewed from the ventral aspect, the brain shows some 

 additional structural features. Beginning again at the anterior 

 end, it will be noted that the olfactory lobes are not so completely 

 fused as on the dorsal side. Lying posterior to the cerebral hemi- 

 spheres in the median line are the heart-shaped infundibulum and, 

 just behind it, the hypophysis which together form the pituitary 

 body. The portion of the brain lying ventral to the optic lobes 



