346 EDWIN S. GOODKICn. 



position of paiied fms^ we find tliat Gegenbanr seems to liave 

 lield that the wliole girdle and fin-slceleton conld move from 

 its place of origin, dragging to some extent the mnscles and 

 nerves with it. He pointed to the collector nerves and rndi- 

 mentary buds as evidence of this actual migration of the 

 ready-formed pelvic fin. This argument has already been 

 dealt with above (p. 340), and will be further answered below. 

 Brans believes that he has proved that actual migration of 

 the paired fins takes place during the development of Acan- 

 thias. His excellent figures, however, afford convincing evi- 

 dence to the contrary. It is obvious that if a fin, in ontogeny, 

 moves as a whole, no one part of it can remain in its original 

 position. If now we compare his figure of the earlier with 

 that of the later stage in the development of the pelvic fin (figs. 

 1, 2, 3 and 4, PI. 22), we find that the muscle-buds and nerve 

 belonging to segment 36 remain throughout in approximately 

 the same position. The neighbourhood of segment 36, there- 

 fore, represents a fixed point. It is true that the fin-fold 

 extends further forward in the earlier stages than it does in 

 the later, and further back in the later than it does in the 

 earlier; but this is due to the fact that the fin develops, on 

 the whole, from before backwards, and undeigoes more reduc- 

 tion in front than behind. The apparent migration of the 

 fin from segments 21-30 to segments 30-39, during develop- 

 ment, is brought about, not by the actual motion backwards 

 of the whole fin structure, but by the concentration of the 

 fin towards a central region, and by the great reduction of 

 its anterior border.^ 



A fin-fold will appear to move, during development, back- 

 wards or forwards, according as there is concentration and 

 reduction, more in the one direction than in the other. 



In agreement with this, it is found tlmt a fin-fold, and 

 its contained muscular, nervous, and skeletal ele- 

 ments, are derived from that region of the trunk 

 which is occupied by the adult fin (see further, p. 360). 



1 I am inclined to doubt the correctness of llie cnunicialion of tlic segments 

 in Brans' figures. No such extensive ap|iareiit niigralion occurs in Sc^'llinm. 



