18 Mr. li. C. Punnett. On the, Composition and Variation* 



The tables bring out clearly the fact that, in both sexes, as the 

 girdle comes to occupy a more rostral position, the number of post- 

 girdle nerves shows a steady and marked increase. This is exactly 

 what, on the migration theory we should expect to find. On the 

 side-fold excalation theory, however, we should expect the position 

 of the girdle to have no effect on the number of the post-girdle 

 nerves. True, contraction of the fin may be going on, but even then 

 there is no reason why the fins which show the greatest amount of 

 contraction should be so regularly associated with a more rostral 

 position of the girdle. The only way in which these facts can be 

 explained on the excalation theory, is by assuming that the pre-girdle 

 excalation which results in a more rostral position of the girdle, is 

 accompanied by a post-girdle intercalation which leads to a greater 

 number of post-girdle nerves. The greater the amount of pre-girdle 

 excalation the greater must we suppose is the amount of post-girdle 

 intercalation. 



C. Correlation between the First Girdle-pier tin (j Nerve and tie number of 



whole Vertebra?. 



That there is a close correlation between the position of the girdle 

 and the point where the whole vertebras end is brought out by the 

 following table : 



Table XIII. 



From this table it will be seen that there is usually an interval of ten 

 segments between the vertebra through which passes the first girdle- 

 piercing nerve, and the last whole vertebra. This is especially well 

 marked in the four most caudal positions of the pelvic girdle, i.e., when 

 the serial number of the girdle-piercing nerve is 37, 38, 39, or 40, the 

 average number of whole vertebrae is exactly or very nearly 47, 48, 

 49, and 50 respectively. 



Having just seen the necessity of assuming post-girdle intercalation 

 to explain the larger number of post-girdle nerves associated with a 



