AMPHIBIA 



21 S 



Cer. hem. 



Cereb. 

 Optic \ 

 Pin. lobe I Med. 



maturity, impelling migration, and producing even somatic and 

 physiological modification, as in the copulatory pad and the 

 copulatory reflex action. 



Nervous System. The medullary canal formed by the 

 folding of the ridges which define the medullary groove 

 anteriorly, and the extension of the canal posteriorly after the 

 cavity has been shut off by the development of the neurenteric 

 canal, is resolved into an anterior brain and a spinal cord. 

 The conversion of these into the adult structures takes 

 place essentially as in 

 the Elasmobranch, the 

 difference arising by 

 degree of development. 



The brain presents 

 clearly defined cerebral 

 hemispheres, or prosen- 

 cephalon. Behind the pro- 

 sencephalon are the thala- 

 mencephalon, the optic 

 lobes, the small trans- 

 verse cerebellum, and 

 the medulla oblongata, 

 which narrows posteriorly 

 to become continuous 

 with the spinal cord. 



The cerebral hemispheres are well developed. They arise 

 as paired outgrowths of the forebrain. Anteriorly the hemi- 

 spheres end in olfactory lobes which are fused. These represent 

 the middle of the area of the forebrain, which is outgrown to 

 form the hemisphere on each side. The rest of the hemi- 

 sphere remains separate, and, fully expanded from the fore- 

 brain, it presents already areas which may be compared with 

 those of the hemispheres of the higher Craniata. The dome- 

 like roof, or pallial region, is separated by a slight horizontal 

 groove, both medially and externally, from the basal region. 

 Internally the lateral ventricles, which end blindly in the 

 olfactory lobes and communicate with one another and with 

 the third ventricle posteriorly, present also horizontal grooves 

 which serve to define the upper from the basal part of the 



Cer. hemtsphe 



Ant.com 

 for. ftonro 



Chiasmi 



mVent 

 .Post.comm. 

 Infu 



Iter 



FIG. 108. Lateral view and longitudinal 

 section of brain. 



