NERVOUS SYSTEM. 



231 



development of the nervous system of the tadpole produced 

 in one day. In the tadpole of the common frog, on the 

 fourth day, (Fig. 97. A.) the spinal chord is perceptibly en- 



FIG. 97. 



larged at its posterior part (a,) and also the medulla oblon- 

 gata. The cerebellum (b,) is scarcely visible, extended across 

 the median plain ; the optic lobes (c,) and the cerebral he- 

 mispheres (e,) are small, narrow, and so far separate longitu- 

 dinally as to expose entirely the intervening optic thalami 

 (d.) On the following or fifth day (Fig. 97. B,) besides the 

 general encrease of the spinal chord (B. #,) the cerebellum 

 (B. b^ is perceptibly enlarged, the optic lobes (B. e,) are 

 proportionately broader and shorter, and the cerebral hemi- 

 spheres (B. e,) encreased in every direction, begin to extend 

 backwards over the optic thalami (B. d.) As the tadpole 

 advances in its development, and the legs and arms are ex- 

 tended from the sides, the posterior and middle enlargements 

 of the spinal chord are proportionately encreased, the cere- 

 bral hemispheres enlarge, and their white fibrous internal 



