210 JOHN BEARD. 



the spinal have no connection with the skin in early stages ; that is, 

 the ganglion is never fused with the skin, and their roots are never 

 connected with gill-clefts or with special sense organs. 



One of the most striking results of these researches is the great dis- 

 tinction of the body of Vertebrates into a gill-bearing region and a 

 non-gill-bearing region ; and at present, with the sharply-defined 

 differences which obtain in the development of the organs of these two 

 regions, attempts to homologise organs in the two different regions 

 would seem to meet with indifferent success. That Balfour was right 

 in regarding the cranial nerves as more primitive than the spinal is 

 probable enough, but at the same time it is very questionable whether 

 the spinal nerves ever had the same primitive characters as the cranial. 



Dohrn's idea that the anus arose from a pair of coalesced gill- 

 clefts may be rejected without more ado, for there seems to be 

 no evidence for it. Not so, however, his mode of regarding the 

 mouth as a pair of coalesced gill-clefts ; that is probably true. In 

 dealing with the relations of head and trunk, the vexed question 

 of anterior roots of cranial nerves crops up, and with it the 

 nature of the head cavities. I have no observations to record on the 

 so-called anterior roots of cranial nerves except on the hypoglossus, 

 which has certainly nothing to do with the cranial nerves, as Dohrn 

 has pointed out. Van Wijhe regarded the hypoglossus as made up in 

 Elasmobranchs of three anterior roots of the vagus. In this point my 

 researches agree with those of Dohrn and Froriep. The hypoglossus 

 has nothing to do with the vagus. 



Froriep's 1 account of the development of the former in Mammalia 

 seems to hold good also for Elasmobranchs. As in Mammalia, the 

 hypoglossus of Elasmobranchs is derived from the anterior roots of the 

 first three spinal nerves. The posterior roots are developed in the 

 embryo, but afterwards abort. I have not figured them, because the 

 spinal nerves really lay beyond the scope of this work. 



As to the head cavities themselves, their persistence in the anterior 

 part of the head may, as other observers have stated, be due to their 

 functional connection with the eyes. That they once occurred in all 

 the segments of the head is probable enough, though with what organs 

 they were originally connected is not so plain. Possibly from their 

 muscular nature, and the apparent absence of sensory elements, even 

 in development, in their nerves, they may have been the muscles of 



1 Op. cit, pp, 5 and 48. 



