CORRELATING CENTERS IN THE DIENCEPHALON. 



present in the floor of the brain ventricle in Amphioxus in a 

 position corresponding to that of the saccus ; a fact which indicates 

 how ancient a structure the saccus is and that it functions in 

 relation to the brain ventricle. In fishes the ciliated cells taper 

 to a point at their outer ends and give rise to nerve fibers which 

 help to form the fiber layer. These cells may be compared with 

 typical primitive nerve cells elsewhere in the brain. The nerve 

 fiber arises from the peripheral end of the cell as in the case of the 

 neuroblasts in the embryo, and the cell-body retains the pos- 

 ition in the epithelial lining which is characteristic of the 



Lobus inferior 



Fig. 143. The efferent tract to the saccus vasculosus in the sturgeon. The 

 drawing represents a sagittal section a little to one side of the median plane through 

 the front part of the saccus and a part of the ventral wall of the inferior lobe. 

 The course and main branches of three fibers are shown, from among the many 

 impregnated in the section. 



germinal cells and many neuroblasts. Well developed nerve cells 

 which retain their epithelial position are very numerous hi the 

 brain of fishes (cf. p. 48). The saccus cells do not produce 

 dendrites but instead bear cilia projecting into the ventricle. It 

 must be supposed that these cells receive stimuli of some sort, 

 whether vibratory or chemical, from the cerebro-spinal fluid. 



