48 Whitney, on the Metamorphosis of the Tadpole. 
vascular system of the perfect gill we must return to the 
living subject. 
The brilliant but perplexing maze of blood-vessels which 
the fully developed gill presents in the ordinary mode of 
looking at it is, in fact, the vascular plexus of which the 
crests are mainly composed. If we select a very transparent 
tadpole, and examine a portion of the crest surface with a 
■f or \ inch power, we find the blood-vessels assuming such 
shapes as are represented in fig. 8. These shapes afford an 
explanation of the change from tufts to crests , as far, at least, 
as the vessels are concerned ; for it is easy to perceive that 
when the blood and the impetus of the heart and arteries 
are withdrawn from the external gills (by their destruction), 
and expended upon the looped vessels of the digit-like pro- 
cesses, how each simple loop becomes extended into the 
tortuous and plexiform shapes observed, thus largely multi- 
plying the vascular surface exposed to the aerating medium. 
We have now to examine the course and connections of the 
large trunks which convey the blood to and from the internal 
gills. 
It was not until it occurred to me to remove in the living 
subject the integument in front of the gill that I succeeded 
in obtaining a clear view of its vascular system. I had pre- 
viously examined a large number of very transparent tadpoles 
by simply looking through the skin, yet found it impossible 
to obtain a clear and satisfactory view of the blood-vessels of 
the inner gill. But by removing the integument in front of 
the gill, so that the latter may be laid bare while the circula- 
tion is still vigorous, a most brilliant and beautiful sight is 
presented, and the observer is astonished that the little film 
he has removed (albeit apparently quite transparent) effec- 
tually obscured the simple and beautiful piece of mechanism 
now before him. This operation can be performed without 
pain. One drop of chloroform effectually destroys the sensi- 
bility of the tadpole without damaging the force of the circu- 
lation. I have seen the latter continue, with very little re- 
duction of force, for two hours, under the influence of one 
drop of chloroform. Should any sign of sensibility return, it 
may be immediately quenched by touching the body with a 
camel-hair brush dipped in the chloroform. 
This dissection, to be successful, is a delicate and difficult 
affair. Difficult, because, on the one hand, by incising ever 
so little too deep, some of the vessels are wounded, and the 
consequent haemorrhage empties the arteries and obscures 
them ; while, on the other, if we do not remove enough of 
the fine tissue between the skin and the vascular plexus, we 
