358 EDWIN S. GOODKIOII. 



by sections^ reconstruction, and whole preparations. Horizontal 

 sections tlirougli the dorsals, at a stage when the procarti- 

 laginoLis radials are just beginning to appear, show the con- 

 cordance quite })lainly (fig. 8). In figures of longitudinal 

 sections, passing obliquely through two pelvic fins (fig. 18) 

 the correspondence is obvious. A reconstruction of one of 

 these pelvic fins, in which the ventral muscle buds are drawn 

 in the anterior half and the dorsal buds in the posterior half, 

 is no less conclusive (fig. 9). The same may be said of a 

 reconstruction of the first dorsal of an embryo 33 mm. long, 

 in which the radials are beginning to appear (fig. 7), and 

 also of reconstructions of both a pelvic and a dorsal fin, in 

 which the cartilage is developing (figs. 11 and 12). 



The contention of Braus (4, 7, see p. 339 above), that the 

 concordance is secondarily established late in development, 

 is utterly at variance with all my observations. 



Concentration. 



The process of concentration in the median fins can be 

 followed on comparing the whole series of stages from the 

 earliest appearance of the buds to the adult condition (figs. 

 1-4, 6, 7, and 11). 



At first the fin rudiment extends over as many segments as 

 produce buds — about sixteen to eighteen. This ideal first 

 stage is, however, never perfectly recapitulated, since the 

 central buds develop first and become slightly concentrated 

 before the other buds have appeared (figs. 1 and 25). 



In later stages the body segments lengthen much faster 

 than the fin rudiment, so that the buds have the appearance 

 of actively growing towards the base of the fin from both 

 sides. As a matter of fact, they probably remain passive 

 during the process. It is possible, however, that they may 

 grow to a slight extent towards the fin, but such a growth 

 would be very hard to prove. On the other hand, they 

 undoubtedly grow outwards into the develo})ing fin-fold. 



Both the fiu-fokl, with its contained muscle-buds, and the 



