MUSTELUS L^,VIS. 195 



clilnxlar cleft^ without there being" any modification of the 

 epithelium at the point in question." The modified epithelium 

 of the anterior diverticulum of his descriptions of the cleft, 

 considered by Wright as a neuro-epithelium, is said to receive 

 some fibres from this pretrematic branch. 



In none of my embryos did the truncus facialis here 

 separate into pre- and post-trematic portions, and it seems 

 to me almost beyond question that Wright mistook the 

 superior postspiracular ligament of the fish, together with 

 a nerve described below, for a pre-trematic branch of the 

 facialis, a part of the ligament, together with the nerve, 

 having, in embryos, almost exactly the position and dis- 

 tribution of the so-called nerve described by him. The 

 modified epithelium of Wright's anterior diverticulum, my 

 dorso-mesial one, received, in all my embryos, a nerve which 

 made its first appearance in sections as a large nerve lying in 

 the middle of the postspiracular ligament at the point where 

 that ligament begins to spread and break up into its three parts. 

 The section of the nerve as it here first appeai'ed in following 

 the series of sections backward was large, and lay close to 

 the truncus facialis, but it was distinctly and definitely 

 separated from that truncus by a small slip of the ligament. 

 Traced posteriorly the nerve ran downward and backward 

 to, and ended in, the modified epithelium of the dorso-mesial 

 spiracular diverticulum. Fibres of it here quite certainly 

 also went to the modified epithelium of the closely adjacent 

 ventro-mesial diverticulum, but this could not be definitely 

 traced because of the dense fibrous tissues that here surround 

 both diverticula. In the section next anterior to the one in 

 which this spiracular nerve thus suddenly appears, there 

 was no indication v/hatever of it, excepting only a slight 

 discoloration of the mesial edge of the postspiracular liga- 

 ment. I was, therefore, at first inclined to consider the 

 nerve as a branch of the truncus facialis, as Wright had 

 done before me. Later I became convinced that it could 

 not be a branch of the truncus facialis — that I can posi- 

 tively affirm, — and I was then first inclined to consider it 



