136 COMPARATIVE ANATOMY. 



central nervous system in the lower Vertebrata. In higher Verte- 

 brates, its double nature persists only in the region of the vertebral 

 column, the two layers becoming fused in the cranial portion. As 

 the brain of Fishes by no means fills up the cranial cavity, a large 

 lymph-space' lies between the dura and pia mater ; this cor- 

 responds to the so-called sub-dural space of the higher 

 Vertebrata. 



A differentiation of the primary vascular membrane of the 

 brain and spinal cord into pi a mater and arachnoid takes 

 place in the higher Vertebrates, and these two layers become 

 separated in those places where there are deep depressions be- 

 tween the individual parts of the brain ; the deeper of these (pia) 



FIG. 109. BRAIN- MEMBRANES OF MAN. (After Schwalbc.) 



DM } dura mater ; Sfi, sub-dural cavity ; A, arachnoid ; PM, pia mater ; GR, gray 

 cortical substance of the brain. 



adheres closely to the brain, and also penetrates into the ventricles 

 in the form of telse choroidese and plexus choroidei, 

 while the superficial one (arachnoid) simply bridges over the 

 depressions. 



No certain explanation can as yet be given of the morphological 

 meaning of the hollow anterior end of the spinal cord in Amphi- 

 oxus, nor of the diverticulum connected with it which opens 

 freely to the exterior on the dorsal surface. 



Fishes. 



The Cyclostomi show a very low condition of the brain, 

 which in many points remains in an embryonic condition. This is 

 particularly the case in the larval condition (Ammoccetes, Fig. 

 110), in which the brain possesses a narrow and elongated form. 

 The individual vesicles lie in an almost horizontal direction 

 one behind the other, and it is of great importance to note that 



