A442 THE ORIGIN OF VERTEBRATES 
individual thus budded off arises not from the gut or hypoblastic 
layer of the parent, but from the surface or epiblastic layer. Such 
gut so formed possesses as efficient digestive powers as the gut of 
the parent. 
The most remarkable evidence of all has been afforded by 
Miss Alcock’s experiments. She examined the different tissues of 
Ammoccetes for the express purpose of finding out their power of 
digesting fibrin, with the result that the most active cells were 
those of the liver. Next in activity came the extract of the lining 
cells of the respiratory chamber and of the skin. The intestine 
itself when freed from the liver-secretion had very little digestive 
power; extracts of muscle, nervous system, and thyroid gland had 
no power whatever, but the extract of the skin-cells possessed a 
powerful digesting action. 
Furthermore, it is not necessary to make an extract of the 
skin in order to obtain this digestive fluid, for under the influence 
of chloroform the skin of Ammoccetes secretes copiously, and this 
fluid thus secreted was found to possess strong digestive powers. So, 
also, Miss Aleock has demonstrated the power of digesting fibrin 
in a similar secretion of the epithelial cells lining the carapace of 
the crayfish. In both cases a very plausible reason for the presence 
of a digestive ferment in a skin-secretion is found in the necessity 
of preventing the growth of parasites, fungoid, or otherwise, especially 
_in those parts where the animal cannot keep itself clean by 
‘preening.’ Thus in a crayfish, in which the esophageal commissures 
had been cut, fungus was found to grow on the ventral side, but not 
on the dorsal carapace. The animal was accustomed to keep its 
ventral surface clean by preening; owing to the paralysis it could 
not do so, and consequently the fungus grew there. In the lamprey 
I found that wherever there was a removal of the surface-epithelium, 
from whatever cause, that spot was immediately covered with a 
fungoid growth, although in the intact lamprey the skin was 
invariably smooth and clean. 
I imagine, then, that this digestive power of the skin arose as 
a protective mechanism against parasitic attacks; it is self-evident 
how a tube formed of such material must abd initio act as a digestive 
tube. 
In yet another respect this skin secretion of Ammoccetes is most 
instructive. The surface of Ammoccetes is absolutely smooth, no scales 
