654 AGNES I. M. ELLIOT. 



nerves of segments now lost ; segments whose myotomes are 

 represented by tlie undifferentiated mesoblast in front of the 

 first existing myotome. 



To sum up the results obtained — 



1. The first spinal nerve with its ganglion and associated 

 myotome is well developed in the young tadpole, and still 

 present, though somewhat reduced, in the tadpole which has 

 developed dorsal cartilaginous arches, and whose limbs have 

 begun to develop. 



2. Two myotomes are present in the 9-millimetre tadpole 

 in front of this segment. Cartilaginous arches appear in 

 connection with these, and fuse with the parachordals, fi'om 

 which they are still distinct in a 20-millimetre tadpole. 

 There is no ganglion in the segment of the first myotome, 

 but there is a rudimentary one in the segment of the second 

 myotome. Both these myotomes and the rudimentary gan- 

 glion disappear by the time that the cartilaginous dorsal 

 arches are present, while the cartilaginous arches corre- 

 sponding to them form the occipital region of the skull. 



3. The vagus arises by numerous roots. The first root 

 has a ganglion, and lies well forward and distinctly separated 

 from the remaining roots. The latter may possibly repre- 

 sent ventral roots of the nerves of the missing post-otic 

 segments, and also of the segment in which the first myotome 

 is developed. 



4. There is a string of true mesoblast lying between the 

 otic sac and the first myotome. This may also be the 

 remains of the tissue of segments anterior to that of the first 

 myotome. 



5. The segmentation of the post-otic region of the skull in 

 Raua agrees completely with that in Necturus. 



My sincere thanks are duo to Dr. Gadow, both for his 

 suggestion of the subject of this investigation, and for his 

 kind interest and advice during its progress. 



