124 THE SALAMANDER 



them was given by Rex in 1 893, in his paper on the blood-vessels of 

 the brain in Triton and Salamandra. Five years later, in 1898, Miss 

 Helen O'Neill made a very careful and detailed study of this subject, 

 taking Salamandra as her type. She describes two membranes, a 

 dura mater — of which there are two layers — and a primary vascular 

 coat. Later Sterzi in a series of papers (1899— 1909) reviews the 

 structure of the meninges of vertebrates in general, and supposes that 

 there is only one primary covering to the brain in Urodeles — the 

 ' Meninx primitiva' — from which may separate a ' Meninx secundaria' 

 (corresponding with O'Neill's primary vascular coat), both of which, 

 in higher animals, give rise to the pia mater and arachnoid. More 

 recently Flexner (1929) has studied the development of the brain 

 coverings in Amphibia in both normal and experimental animals and 

 has come to the conclusion that in Amblystoma (taken as a type of 

 Urodeles) there are three membranes investing the brain, the dura 

 mater, pia mater, and arachnoid, and that the last develops from the 

 inner surface of the dura. 



Thus we are faced with three competent observers describing 

 respectively two, one, and three primary meninges in the adult 

 Urodele. The differences between them are largely those of inter- 

 pretation of homologies and mode of origin, since each admits three 

 membranes. 



Miss O'Neill's work was done under the supervision of Professors 

 Wiedersheim and Gaupp and is the most detailed as far as Sala- 

 mandra itself is concerned. It has been thought advisable therefore 

 to adhere to her interpretation pending a more satisfactory and deci- 

 sive solution of the whole problem. 



Following Miss O'Neill's terminology, the two membranous en- 

 velopes surrounding the brain and spinal cord are: 



(i) The dura mater on the outside. A firm tough membrane, con- 

 sisting of connective tissue-fibres with flattened cellular elements 

 between them. It is slightly pigmented, the pigment cells being of 

 the stellate variety. In a few special places, to be enumerated later, 

 the dura mater splits so as to form two layers separated by a space, 

 the interdural space. The outer layer, next to the bone of the cranium 

 or vertebral column, is the periosteal layer \ the inner layer, lying 

 nearer to the brain or spinal cord, is the neural layer. 



(ii) The inner envelope or primary vascular coat, which carries 

 the blood-vessels. It lies next to the neural tissue itself and 

 corresponds with the pia mater of most authors. According to 

 Miss O'Neill, two layers are distinguishable: namely, an inner, 

 more or less homogeneous, firm layer in which cellular elements 



