22 Mr. R C. Punnett. On the Composition and Variations 



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 



