THE TISSUES. 



23 



cation of the epithelial tissue, due to its differentiation. 

 Like it, glandular tissue always consists of cells. In the simplest 

 stage individual cells in a layer of epithelium become secreting-cells, 

 and function as gland-cells, by forming and secreting a substance 

 such as is not produced by the other cells. In this way uni- 

 cellular glands arise. They either retain their 

 original position between the other cells of the 

 layer or sink beneath the level of the epithelium, 

 and open between the other cells by a fine duct 

 formed by the membrane of the cell (Fig. 7). 

 If the secreting surface be increased without the 

 general epithelium taking any share in it, the 

 sunken epithelium must increase in size, and so 

 give rise to structures which are more or less 

 separated off from the epithelium, such as pits, 

 sacculi, or ceecal tubes ; and these may be again 

 complicated by fresh growths. The tissue lying 

 beneath the primitive epithelial layer forms en- 

 velopes for these pits as they grow; but it con- 

 tinues to have the same relation to them, however 

 complicated in form the ramifications and similar 

 proliferations of the epithelium may be, as it pre- 

 viously had to the simple even layer of epithelium. 

 Thus the gland in its simplest form appears 

 as a depression of the epithelium into the sub- 

 jacent tissue. In the more distinct forms of glands 

 there is a further differentiation of the cells which 

 form the gland. The constituent cells of the 

 gland become separated into those which secrete 

 and so represent true gland-cells, and those which connect the 

 secreting portion of the gland with the still indifferent epithelial 

 layer. These, in contradistinction to the secreting portion of the 

 gland, form the epithelium of the duct. 



Fig. 7. Unicellular 

 glands. Anterior 

 salivary glands of 

 the ant (after Stein). 



Connective Substances. 



§ 20. 



The phaenomenon which in epithelial tissue leads to the formation 

 of homogeneous membranes may, by being extended over the whole 

 periphery of every cell as well as by continued repetition, become of 

 greater importance. Even in the epithelial tissue we often meet 

 with a fine intermediate layer, the cement-substance. As the sub- 

 stance which is differentiated from the protoplasm of a number of 

 cells gradually increases between the cells containing unaltered 

 protoplasm, the cells become separated from one another, and a 

 distinction is made between the cells which form and the inter- 

 cellular substance which is formed. A number of very different 



