40 DR. A. MILNES MARSHALL. 



regards its discovery as "one of the most remarkable results of 

 his researches upon the Elasmobraiich neivous system." I 

 am not aware that it has been noticed by any other observer. 



In ligs. 23 and 26 a nerve {co7)i) is see:i running back- 

 wards from the posterior root of the vagus close to its 

 point of origin from the brain. This runs backwards, and 

 becomes directly continuous with the longitudinal commissure 

 just described as connecting together the posterior roots of 

 the spinal nerves ; it is manifestly an anterior continuation of this 

 latter. 



It is shown very well in fig 21, which represents a longitudinal 

 section through the hinder part of the head and the cervical region 

 of a ninety-six hours' chick. The commissure {com) presents two 

 ganglionic swellings (IX, X) at the points of origin of the glosso- 

 pharyngeal and vagus nerves. It also presents smaller swellings, 

 not shown in the figure, corresponding with the posterior roots 

 of the spinal nerves. It is shown also in fig. 25 [com). 



The existence of this commissure is a very remarkable fact. 

 I have not followed it late enough to know its ultimate fate. In 

 Elasmobranchs, according to Balfour, " it becomes gradually 

 thinner and thinner, and finally ceases to be observable ... I can 

 only conclude that it gradually atrophies, and finally vanishes 

 without leaving a trace.'''* 



I have failed to trace the commissure further forwards ; but 

 the forward extension of the anterior root of the glosso-pharyn- 

 geal, already noticed, is of interest as indicating a probable con- 

 tinuation, and I have already suggested that the ophthalmic 

 branch of the fifth nerve is very possibly the part of the com- 

 missure in front of the fifth nerve. If this is so, it is the only 

 part that we know to persist in the adult condition. 



Since the intervening portions of the ridge persist, at least for 

 a time, in the posterior part of the head as a definite structure, 

 there is certainly a presumption in favour of a similar persistence 

 of the anterior part of the ridge, which is at places of enormous 

 size in its early stages (fig. 7, ?)i). Its enormous size at this 

 part of the brain may, however, be due in part to the general 

 hypertrophy to which the anterior part of the neural canal is 

 subject. 



' Loc. cit., pp. 471—426. 



