Page:Proceedings of the Royal Society of London Vol 69.djvu/26

18

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.

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.

1st girdle- piercing nerve.

Girdle whole 45.

4G.

47.

48.

49.

50.

51.

Average.

Average number of post-girdle whole vertebrae.

35

6

13

4

_

_

_

43-91

10-91

30

2

24

22 10

2

46 76

10 76

37

2

15

21 ! 12

4

47 -0-2

10-02

38

4

16 19

6

2

2

47 -S3 9-83

39

5 4

48-8$

9-88

40

2

.50 -00

10-00

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