190 



COMPARATIVE ANATOMY 



appearance of a lymph-space in the primitive meningeal membrane, 

 dividing it into an outer dura mater spinalis and an inner primitive 



pia mater. There is thus a pi //</// /'//I or <'j>/</n./-I xjxtir, external to 

 the dura., and a xitlitlii.nil .syvmr between it an<l the primitive pia 



(B). 



B 



iiiiitire pin mater 



space 



Fi(!. 146. THE INVESTING MEMBRANES OF THE SPINAL CORD IN THE CHIEF 

 VERTEBRATE GROCPS. A, Fishes ; B, Amphibia and Sauropsida ; C, Mammalia. 

 Semidiagrammatic. (After Ster/i.) Riickenmark, spinal cord. 



In Mammals, the pia is considerably thicker, and the lymph 

 spaces which it encloses are more marked : it becomes separated 

 into two layers, the inner of which gives rise to the vascular 

 secondary or definitive pia, and the outer to the arachnoid. 



