29 



Most males show this and this is also found in the female, though not 

 so generally and always very slightly marked. 



This development of vascular filaments on the pectoral fins was 

 only found in breeding males — i. e. in those which had a full growth 

 of filaments on the pelvic fin. In all the specimens examined by me, 

 in which the growth was present at all on the pectoral fin, it was 

 approximately equally developed on the fins of both sides. 



/-. 



St*^' 



All the figures are orientated alike, showing the fins in their 

 Datural positions, pointing horizontally backwards. It will be seen by 

 the figures that in the pectoral fin the filaments grow out from the 

 ventral border, and in the pelvic from the dorsal. (In the case of the 

 pelvic fin the attachment of the filaments is slightly on the inside of 

 the dorsal border, and the length of the filaments and the direction 

 in which they are set causes many of them to project ventral to the 

 axis of the fin.). 



Thus the filaments grow out from morphologically the same borders 

 of the fins, for the rudiments of the anterior and posterior limbs in 

 the embryo undergo a rotation in an opposite direction in order to 

 bring about the backward direction of the fully developed limbs i) so 

 that the ventral border of the one is homologous with the dorsal border 

 of the other. 



One interpretation that might be put upon this abnormality is 

 that it belongs to a well known class of variations called by Bateson 

 homoeotic, of which the most striking examples are to be found among 

 the Arthropoda, where many cases are known of the appendage of 



1) Graham Kerk, loc. cit. 



