SPINAL XEHVES OF THE FROG. 117 



to produce the complex known as the urostyle, several of 

 the most posterior spinal nerves are eliminated in the 

 process. The condition of the nerves of this specimen 

 may, therefore, be a reversion to an ancestral condition, 

 before the coccygeal nerve became reduced to its present 

 subordinate function. Or, as to me seems more probable, 

 the sciatic plexus is becoming postfixecl, and is incor- 

 porating the coccygeal nerve instead of dropping it, as 

 Adolphi and Miss Sweet contend. The post-fixation of 

 the sciatic plexus is also borne out by the condition of 

 one of the other two frogs now described, in which the 

 plexus also exhibits a backward movement. It must be 

 borne in mind, however, that Adolphi and Miss Sweet 

 have arrived at an entirely opposite conclusion, but their 

 statistical method appears to me to be open to some 

 objection, especially in the light of recent work on the 

 number of fibres in the Frog's spinal nerves. 



The second case, a R. esculenta, illustrates the compound 

 nature of the urostyle. On the left side the brachial 

 plexus is not quite normal, and on the right side the sixth 

 nerve was missing. This may have been removed by the 

 student before the specimen came into my hands, but 

 there were no obvious signs that it had ever existed. The 

 nerves vii., viii., ix. and x. are in all essential respects 

 the same as in the previous specimen, that is to say, the 

 coccygeal nerve is much larger than usual, and passes 

 over entirely, with the exception of a small bundle of 

 fibres as shown in the figure, into the sciatic plexus. In 

 addition to these, however, an eleventh nerve is present, 

 emerging from the urostyle at about the posterior 

 extremity of its anterior third. The vertebral column 

 was quite normal, and the urostyle did not appreciably 

 differ from what may be considered its normal relative 

 length. The extra nerve was very small, and was not 



