22 COMPARATIVE ANATOMY. 



are differentiations of tte protoplasm, and that their movements are 

 due to the same cause as the movements of the protoplasm. This 

 indication of their identity with protoplasm cannot he seen in the 

 more differentiated forms of cilia. 



Another differentiation may be seen on the outer surfaces of 

 many epithelial cells. A membrane, instead of being formed by a 

 change of the whole periphery of the superficial layer of protoplasm, 

 may be formed on a definite portion of it only : in this case it is more 

 highly developed, and may lead to a partial thickening of the outer- 

 most layer of protoplasm. In short, a layer of varying thickness 

 of a substance different from, but as a rule still connected with, 

 the protoplasm, forms on the outer face of each cell. Homogeneous 

 membranes — cuticles — are formed by the further differentiation of 

 the substance thus secreted in a layer from the protoplasm of the cells ; 

 that is by the part formed from each cell becoming more intimately 

 connected with the layers formed by the cells around it than with 

 its own cell. Where these layers are laid down irregularly and 

 gi'adually undergo other changes, by means of which each fresh 

 addition can be distinguished from the preceding one, they become 

 laminated. The more the substance of which these cuticular struc- 

 tures are composed differs from the protoplasm of the cells which 

 have deposited it, the more difficult is it to make out any passage of 

 the protoplasm into it, and the more distinctly is the formation of 

 cuticles seen to be a process of secretion. When the cuticle is not 

 formed regularly on the surface of the separate cells, protoplasmic 

 processes project from the secreting cell-layer into the secreted layer, 

 which are then traversed by corresponding canals (pore- canals) : 

 these are ordinarily very fine. These cuticles differ greatly in con- 

 sistency, and present every intermediate step between softness and 

 extreme hardness. They are often converted into organs of support, 

 when they are very firm ; in which case they ordinarily consist 

 of a substance known as " chitin." These chitinised cuticles are 

 very common in the Invertebrata. 



§19. 



The secreting activity of the cells of large epithelial layers may 

 give rise to liquid, or even to gaseous bodies. The epithelia there- 

 upon enter into new relations to the economy of the organism; they 

 no longer produce substances destined to build up the organism, 

 but they present an intermediate step towards that condition of 

 epithelial structures in which parts of the epithelium enter into the 

 formation of a tissue of definite function — glandular tissue. As 

 there is always a direct connection between the aggregation of 

 cells which form the secreting organs, or glands, and the epithelium, 

 which either persists permanently, as in the majority of glands, or 

 which is at any rate present when they are first formed, this 

 glandular tissue is seen to be nothing more than a modifi- 



