120 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



intercalated somewhere.* Impressed by the amphicoelous 

 centrum of the vertebra labelled 7 in fig. 3, I was at first 

 inclined to believe that the extra vertebra must be either 

 8 or 9. Brit this view involved such confusion in 

 homologising the "spinal nerves" .that it had to be 

 abandoned. But if we suppose that the intercalated 

 vertebra is 6 + and the intercalated nerve vi + , all 

 difficulties at once vanish, and the result is a frog 

 absolutely normal in its nerves, and only exhibiting an 

 abnormality in its vertebral column for which we have 

 many parallels. I therefore adopt this view on the 

 following grounds : — 



1. The nerve vi + does not, as it should, enter into 

 the lumbo-sacral plexus. There are thus five NN. 

 abdominales instead of four, and the distribution of these 

 shows the extra one to be that labelled vi + . 



2. The fact that the vertebra numbered 7 is amphi- 

 coelous does not necessarily prove that it is the morpho- 

 logical eighth. t It shows that the amphicoelous vertebra 

 should be the penultimate one, and the vertebra 7 is the 

 physiological (though not the morphological) penultimate 

 vertebra, since 8 and 9 are immovably fused. 



3. The lumbo-sacral plexus is absolutely normal, both 

 in its formation, branches and distribution, whereas if one 

 of its factors had been an intercalated nerve, we are 

 entitled to expect that it would have had some appre- 

 ciable effect on the plexus. 



4. Howes has described in a normal nine vertebra frog 



*I must not be understood, when the term "intercalation" 

 is used, to imply that the intercalated structure is a neomorph, as Baur 

 believes. My position is rather that of G. H. Parker (No. 18). 



+ Lloyd Morgan (No. 17) describes two eighth vertebrae of 

 R. temporaria as not being amphicoelous, and mentions that he 

 has seen this abnormality before. 



