No. 431.] PAIRED LIMBS OF THE VERTEBRATES. 845 
rim alone is in advance of the gravity center. Thus, as this 
center passes backward, the posterior margin of the ventral 
fin is found to follow it pari passu, and, as effecting a counter 
balance, like equal weights placed in opposite scale pans, the 
anterior rim of the pectoral passes headward. 
Again, contrasting the embryos shown drawn to scale in 
Fig. 1, and referring now to the position of the external gill 
openings, we note that there is a marked tendency for these 
openings to pass backward. In the earlier specimens all are 
widely interspaced and the last is well in front of the fore rim 
of the pectoral; in the next embryo the openings are drawn 
together, altogether somewhat tailward ; and in the latest, as 
far at least as to the second opening, the series have taken a 
position hindward of the rim of the pectoral. 
' More concretely (measurements taken from the specimens) : 


| 
PERCENTAGE OF LENGTH FROM SNovT Tip. | 
| PERCENTAGE OF TAIL- 
WARD MIGRATION. 

GILL OPENING. 





Stage Ir. Stage 2. Stage 3. 
I 11% 13% 16 % 5 % 
y 19 % 19 % 23.5 % 4-5 % 


In other words, as far as these openings are concerned, and 
with them the outer part at least of the gill canal, it is evident 
that in the older stages a definite migration has taken place, to 
a degree that, in terms of the total length of the animal, the gill 
Openings come to occupy a position in the body wall which is 
largely behind that of the earlier stages. Numerically consid- 
ered, the interval between gill openings I and V is about 8 per 
cent of total length in stages 1 and 3; but in the latter the 
region of the gill openings is nearly 5 per cent farther tailward ; 
therefore, in this stage the openings occupy a position in the 
body surface which is about five-eighths new, z.e., never before 
occupied by gills. From this it follows that the gill region, at 
least in its outer part, shows no affinity during proportional 
&rowth with the neighboring region of the pectoral fin. In fact, 
from an early stage onward, they are evidently growing in oppo- 
Site directions. 
