192 



GENERAL BIOLOGY 



varied and complex, and are ever effected in new combina- 

 tions. Hence there is a correspondingly large proportion 

 of the regulative cells of the body, the nerve cells, located 

 in the neural tube. The most significant new feature of 

 cell grouping found among vertebrates is the aggregation 

 of nerve cells at the forward end of the neural tube to 

 form the brain. The cord widens on entering the skull 

 into the medulla. On its dorsal side a thin roofed 



a 



m 



n 



Fig. 118. Diagrams of the brain of the salamander, zo, 

 dorsal view; x, ventral view; y, lateral view and z, dia- 

 gram of the continuous internal cavity in dorsal or 

 ventral .view; a, olfactory lobe; b, cerebral hemi- 

 spheres" c, pineal body; d, thalamencephalon; e, optic 

 lobes; /, cerebellum; g, medulla, h, spinal cord; i', in- 

 fundibulum; /, hypophysis; ^.lateral ventricles; /, third 

 ventricle; w, optic ventricle?; n, fourthrventricle. 



V-shaped slit, called the fourth ventricle, exposes the cen= 

 tral cavity that extends in fact throughout its length. 

 A transverse ridge of nervous tissue at the front of the 

 fourth ventricle upon the dorsal side is the cerebellum. A 

 pair of rounded swellings just in front of the cerebellum are 

 the optic lobes. The pair of large oblong lobes at the front 

 are the cerebral hemispheres. These and other parts exter- 

 nally visible may be located by reference to figure 118. 

 Their relations to each other will be considered when their 

 development is studied (p. 198). 



