Raymond C. Osburn 185 
ectoderm and lies not only external to the body cavity but also to the 
larger blood-vessels. This condition is continued into the adult where a 
fo) 
portion of the last gill-arch is overlaid by the pectoral girdle. 
MIGRATION OF THE PECTORAL ARCH. 
The question of the migration of the paired limbs is a vital one for 
the gill-arch theory since the origin of the limb girdles from gill- 
arches involves their translation from the branchial region. ‘That a 
certain amount of shifting of position may occur during the development 
of the paired fins no one will deny, but that the same sort of shifting 
may also occur in the growth of the unpaired fins is equally true. Such 
migration is always comparatively slight, and it may take place either 
forward or backward. Moreover, the shifting of the paired fins is corre- 
lated with that of the unpaired fins (Dean, 02; Punnett, 04) and with 
that of the center of gravity of the body of the developing embryo 
(Dean, 02). With regard to the supposed early backward migration 
of the pelvic described by Braus, 98, the writer has already (o6b) sug- 
gested a different interpretation, and Goodrich, 06, has stated, as a 
result of careful observations on Scyllium, that the facts of development 
will not bear the interpretation given by Professor Braus. The abortive 
muscle-buds anterior to the pelvic fin cannot, then, represent the path 
over which the pelvic fin migrated to its present position, but, like those 
in front of and behind the unpaired fins and even behind the pelvic itself, 
they represent muscles which once functioned before the base of the fin 
became as constricted as in recent adult sharks. The attempt to explain 
the abortive muscle-buds behind the pelvic fin as due to a secondary for- 
ward migration of that member (Punnett, 00) is a reductio ad absurdam 
of the migration hypothesis. In their earlier appearance the anterior 
abortive muscle-buds merely follow the law of all such serial structures 
of the body, in that the most anterior are the first to arise. 
The presence of a collector nerve in the anterior part of the pelvic fin 
cannot be considered as evidence of migration of the fin since such struc- 
tures are now known to occur in the posterior part of the same fin and in 
the unpaired fins, both posteriorly and anteriorly. (Cf. Maver, 86; Pun- 
nett, 00; Braus, 04; Osburn, 06b; Goodrich, 06.) 
Forward migration of the pectoral fin.—By the biometric study of a 
very complete series of embryos of Cestracion, Dean in 02 showed that 
the pectoral fin as a whole shifts forward during development, instead of 
backward as the gill-arch theory would require. The present writer has 
investigated the internal structures of the pectoral fin in this form and 
