442 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 Alcock 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 oesophageal 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 ab 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 



