Page:Proceedings of the Royal Society of London Vol 69.djvu/30

22 is due to migration of the fin, we must explain the presence of the posterior collector in the embryo as being due to a secondary rostral wandering of. the fin. In the case of Mustelus ((12), p. 348), it was shown that a portion at any rate of the nerves forming the posterior collector gave up this condition later, and ran down to the fin as separate nerves. Now in the two male embryos, E and F, nerves 9, 10, and 11 of the post-girdle nerves form a posterior collector. From Table III we learn that the average number of post-girdle nerves in the male is 9 '45. Consequently it is not unlikely that the more rostral of the nerves forming the posterior collector in the embryo will afterwards give up the collector condition and run separately to the fin. Such a state of things would, as in Mustelun, be an argument for regarding the collector state as more primitive than the condition in which the nerves run down separately which latter state is to be regarded as the more primitive on the side-fold theory. Much stress, however, cannot be laid on this OAving to the small number of the embryos examined.

Concerning the question of ontogenetic migration of the girdle no answer can be given in the case of Amnthias ndyarix. When we take into account the great amount of variation which occurs in the adults, it is obvious that in order to obtain a satisfactory answer it would be necessary to determine the serial number of the nerves piercing the girdle in a large number of embryos, and to compare the mean with that of a large number of adults. Such a course with the present methods of investigation would involve a labour of some years. Consequently I have left it unattempted. In the case of Mmtelns a certain amount of evidence was collected which tended to show that no such process occurred ( (12), p. 348).

Apart from the case of Torpedo narcr, where both Brans and M oilier have agreed in considering that some such process occurs in the pectoral girdle ( (11), p. 589 and following), the former has attempted to prove that ontogenetic shifting of the fin occurs in Spina.c niger.

The evidence, however, is open to criticism since no account is taken of variation. In support of such ontogenetic migration Braus adduces two pieces of evidence : (a) during embryonic development a shifting occurs in the relation of the muscles to the nerves of the fin; e.g., a nerve which is an embryo of 2 '6 cm. supplies muscles IX and X comes in an embryo of 3'2 cm. to supply muscles VIII and IX, and so on ( (11), p. 568) ; (/;) " die Nervenkaniile der Gliedmassengiirtel schliessen wahrend der verschiedenen Phasen der Entwicklung verschiedene seriale Nervens-tamme ein " ( (11), p. 588). In a table on p. 620 Braus gives diagrams showing the condition of the plexus in adult and in four embryos of different stages after the formation of the fin skeleton. He finds that in embryos of 31 '5 and 32 '0 mm., the girdle-piercing nerve is 29, in an embryo of 40 mm. it is 30. whilst in the adult again it