No. 431.] PAIRED LIMBS OF THE VERTEBRATES. 843 
increases greatly in size, measured antero-posteriorly, from 
6 per cent of length to about 24 per cent, its anterior and 
posterior margins undergoing somewhat different fates. The 
anterior rim of the fin passes forward, suddenly in earlier stages, 
slowly in later ones, altogether a distance of over 12 per cent 
of the total length, a distance considerable enough, I take it, 
to entail important adaptive changes in the structures of the 
encroached-upon region, especially in view of the fact that this 
fin margin grows forward as the gravity center passes back- 
ward. (Cf. also infra, re the continuous hindward growth of 
the outer gill slits.) Interesting in this connection is the 
slight variant in embryos of about 80 mm. ; here, the gravity 
center shifting a little forward, the anterior fin margin passes 
backward, thus indicating a nice adjustment in the mechanical 
relations between them. Ina general way the advance of the 
anterior rim of the pectoral is at the same pace as the retreat 
of the center of gravity. The behavior of the hinder rim of 
the pectoral shows a less close adjustment to the gravity center. 
In early stages this fin rim passes forward (about 11 per cent 
between stages of 15 mm. to 56 mm.) to about the transverse 
plane of the center of gravity ; thereafter, however, it grows 
hindward, gradually increasing the distance from the gravity 
center (11 per cent) (and from the anterior fin rim (total, 24 per 
cent)). By this noteworthy hindward growth it comes in the 
older stages to overlap somewhat the anterior rim of the 
ventral fin. 
The ventral fin is conservative in its position during growth: 
measuring about 6 per cent of the total length in earlier stages, 
it increases to but 12 per cent in latest ones, a gain in per- 
centage of but 6 per cent as against 18 per cent in the pec- 
toral. Like the pectoral, the entire fin passes slightly forward 
in younger stages, then slightly backward. The anterior rim 
remains almost fixed with reference to the body length, the 
posterior passing slightly backward to a degree corresponding 
almost exactly with the shifting of the gravity center. Thus, 
in an embryo of about 80 mm. the center of gravity is 18 per 
cent in advance of this rim, in one of 115 mm. 19 per cent, 
in one of 145 mm. 19 per cent, of 187 mm. Ig per cent, a 
