410 TEGUMENTARY ORGANS 



we are at present concerned, arose from this coat, while the main 

 thickness of the mantle is the product of the metamorphosis of the 

 ■subsequently developed ecderon. 



From all that has been said, I think it results that the Ascidian 

 test is formed from the ecderon of the animal by a process of con- 

 "version which consists in the deposit, through its periplast, of cellulose, 

 and a coincident morphological change which may result in the 

 .production of a tissue essentially resembling either cartilage, bone, 

 connective tissue, or even dentine ; and that, therefore, an attentive 

 study of the integument in this class alone is sufficient evidence that 

 mere structure is no proof of the ecderonic or enderonic nature of any 

 given organ. 



Integument of the Vertebrata. — In these animals there are two 

 classes of integumentary organs, differing in structure, chemical 

 composition, and mode of development. These are, ist, the horny 

 and glandular tegumentary organs produced by the conversion of the 

 cellular ecderon ; and 2nd, the calcified tegumentary organs which 

 appear very frequently to be developed by a process of excretion. 



I. Conversionary horny organs. — If a section be made of the 

 integument of any mammal, it will be seen to be composed, leaving 

 ■out of view its various appendages, of two principal portions, the 

 •enderon or derm, and the ecderon or epidermis. The latter, separated 

 by a more or less distinct transparent line from the former, is internally 

 composed of a homogeneous soft substance, in which are dispersed 

 numerous oval or rounded endoplasts, set more or less perpendicularly 

 to the surface of the enderon. Further outwards, they gradually 

 become more distant and a cavity is developed round each, so that 

 the ecderon becomes distinctly cellular. Still more externally the 

 ■cellular periplast becomes changed in composition, being converted 

 into a denser horny substance, and the change usually takes place so 

 suddenly that the horny external portion (epidermis) is sharply 

 marked off optically, and can be readily separated mechanically and 

 chemically, from the internal unaltered soft portion, the rete Malpighii. 

 The cell cavities at the same time become flattened, and by degrees 

 almost obliterated, apparently by the pressure of the subjacent 

 growing tissue ; but the endoplasts remain, and may always be 

 detected if the horny layers are distended and rendered transparent, 

 by the action of acids or alkalies. The horny stratum of the 

 epidermis is therefore the result of the conversion of the walls or 

 periplast of a whole layer of the cells of the ecderon into horn. 



The hard structures of nails, hoofs, and horns {i.e. horny sheath of 

 the horns of Ruminants) are developed in exactly the same manner ; 



