58 

 5. Supporting Kleme'itu, 



a. ^peidi^mj. — " The spenlyma of ths obloni^ata constitutes 

 the menabrana It-nitms i^iterna throut^Jiout, of course, bat the 

 degree of iif fereatiation covers a /»iJe raa^.e. The simplest 

 condition is found in th^ tela choroidea, for tne cells here 

 io not give origin to ependyoial fibres. This might be callei 

 the epithelial phase of develop^nent. 



The first step in the differentiation of epeniyma occurs 

 in the dorso-lateral region of the oblongata, embracing the 

 tuberculuni acusticum anl the adjacent part of the general cuta- 

 neous nucleus. The ependymal cells are rather narrow here, 

 giving a larger number per surface area. F'rom the tip of each 

 cell, a very sleniar epeniymal fibre pushes its way through 

 the nervous fnatter to the limitans externa. The fibre does 

 not branch, and its general coarse is a straight one, although 

 the details of its path are very irregular indeed, sharp turns 

 of small size occurring along its whole length. While the 

 dia-neter of the fibre as a whole is quite slender, its size 

 is increased at irregular intervals by bea3s and snail knobs. 

 Consult H'ig.S. 



A slightly more advanced condition is to noted for the 

 ependyma of the ventral region. The ependymal cells in the 

 floor of the fourth ventricle are larger than those described 

 above, and their fibres are also much stouter. The general 

 course of these fibres is shown in Kg.S.ep.; and the details 

 of two fibres are given in Fig. 9. It is to be noted that the 



