LATER DEVELOPMENT OF THE FROG. 649 



connected with these segments : a pre-occipital, which fuses 

 with the otic capsule, and an occipital, which forms the 

 occipital region of the skull. 



Dr. Gadow suggested to me that, in view of the absence of 

 the first spinal nerve in the Annra, I should cut a series of 

 sections in the neck region of the developing frog to see what 

 is the embryonic condition in this region — whether the first 

 spinal nerve is present in the embryo, and what other traces 

 there are of the disappearance of segments in this region. 



Fig. 1 is a horizontal section of this region in a 9-milli~ 

 metre tadpole. 



The vagus is seen arising by a number of roots — five in 

 this section with traces of a sixth — of which the first is larger 

 than the rest and ganglionated, and there is a distinct gap 

 between the first and the succeeding roots. 



The latter have no ganglion-cells, and this fact, together 

 with their more posterior position, suggests that they are not 

 homologous with the first, and that they may be ventral roots 

 of spinal nerves drawn into the head, and running forward to 

 join the original cranial vagus. 



In another section there are as many as seven roots to the 

 vagus. 



The third myotome is the first visible in this section, and in 

 front of it lies a string of mesodermic tissue with elongated 

 nuclei running from the anterior myotome to the otic sac. 



If we follow the series of sections we can trace two myo- 

 tomes in fi'ont of this one. In fig. 2 the second is shown, and 

 in fig. 3 we have the complete series. The first is smaller 

 than those posterior to it, and extends forward almost to the 

 otic sac. There is, however, some mesoderm anterior to the 

 first myotome, but it does not give rise to a muscle plate, 

 thongh in a similar section of an animal of about the same 

 age I saw signs of a muscle fibre in this region. We may, I 

 think, fairly look on this mesoderm as representing at least 

 one or more somites in front of that from which the first 

 myotome is developed. 



The second myotome has a trace of a rudimentary ganglion 



