NOTES ON THE DEVELOPMENT OP THE NEWT. 249 



Sagemehl fl8) derives the spinal nerves in the Frog from a 

 neural ridge, but states that they become detached later from 

 the spinal canal, and subsequently joined to it by the dorsal 

 and ventral roots. Bedot (5) states that in the Newt the 

 connection is never broken, and our researches lead us to agree 

 with him on this point. 



HofiFmann (13) describes the spinal nerves in Teleosteans as 

 grovcing from a neural ridge, but appears to think that the 

 cranial nerves, which arise before the neural canal is closed, 

 are, partially at least, derived from the adjacent epiblast. 



O. Hertwig (10), in a few scattered observations on the 

 spinal nerves of the Frog, is inclined to support His' view. 

 More recently the theory of the derivation of the whole or 

 greater part of the cranial nerves from the epiblast has been 

 supported by Mr. Spencer (21) and Mr. Beard (4). This view 

 is a revival of that held by Gotte (9). Mr. Spencer asserts 

 that the whole nerve, including root and ganglion, is, in the 

 Frog, split off from the nervous layer of the epiblast. If this 

 be so, all the branches must ultimately be derived from the 

 same source. Mr. Beard confirms him in this statement, and 

 figures one section showing a thickened mass of epiblast con- 

 tinuous dorsally with the still open neural canal, but there is 

 nothing to show that this thickening becomes a nerve. Such 

 a split, as is figured between it and the external layer of epi- 

 blast, very often occurs in imperfectly preserved specimens. 

 We find no such thickenings in Newt embryos of similar 

 stages, a typical section of which is shown in fig. 15, and our 

 observations on the Frog lead us to doubt the accuracy of Mr. 

 Spencer's account. We have attempted to show that it is, at 

 all events, not universally true for Amphibia, as Mr. Beard 

 assumes. 



Mr. Beard has described in Blasmobranchs (4) a fusion of 

 the typical cranial nerve with the sense organ of its segment. 

 This corresponds with the dorsal fusions found by us in the 

 Newt, The ventral fusion of the nerve with the gill-cleft, as 

 described above in the Newt, corresponds to the second fusion 

 found by van Wijhe in Elasmobranchs (22), and to the ventral 



