LATER DEVELOPMENT OF THE FROG. 653 



Rana therefore entirely agrees with Necburus in the seg- 

 mentation of the head. There are two post-otic segments 

 giving rise to the fii'st and second myotomes, and in front of 

 this a mass of mesoblast, which may represent one or an 

 indefinite number of disappearing segments. There is no 

 ganglion connected with the first myotome, and an extremely 

 rudimentary one with the second. 



The neural arches between the first and second and second 

 and third somites persist, and form the occipital region of 

 the skull. They are homologous with the pre-occipital 

 and occipital arches which Miss Piatt found in Necturus. 



In the 9-millimetre tadpole the third myotome is of full 

 size, and so is the corresponding ganglion. But they have 

 already begun to show signs of diminution in the tadpole, in 

 which the dorsal cartilaginous arches are present (see fig. 5). 

 The nerve of this segment is the missing first spinal nerve of 

 the adult. 



If the division between head and trunk is homologous in 

 Selachians and Amphibia, as is generally assumed, a number 

 of post-otic somites must have entirely vanished in the 

 Amphibia, leaving no obvious trace at any stage of develop- 

 ment. For in the Selachians Brans has demonstrated seven 

 spiuo-occipital segments, and in the Amphibia we have traces 

 at most of three, as shown for both Urodela and Auura. 



The curious origin of the vagus may point to a possible 

 reminiscence during development of these otherwise totally 

 missing segments. The difference between the first and the 

 succeeding roots, the presence of ganglion cells on the 

 former and their absence on the latter, and the union of all 

 the posterior roots into a common trunk, which passes 

 forward to join the first root, are, I think, indications of a 

 difi^erence of origin of the posterior group. The position of 

 the first myotome opposite to these roots, together with the 

 absence of any trace of nerve connected with it, suggests at 

 once that its nerve may have run forward to join this group ; 

 and it is easy to carry the suggestion further, and see in the 

 more anterior roots of the posterior set a rudiment of the 



