26 



ZOOLOGY 



SECT. 



of tissues in which the cells have coalesced into an aggregation in 

 which cell boundaries have disappeared and the nuclei alone 

 indicate the originally separate elements : such a structure is termed 

 a syncytium. Other tissues, again, though originating from cells 

 or by the agency of cells, consist in greater or less measure of 

 non-protoplasmic matter formed between the cells. Tissues com- 

 posed entirely of cells take the form, for the most part, of membranes 

 covering various surfaces, external and internal. Such membranes 

 are known under the general name of epithelia (Fig. 11) ; they 

 may consist of a single layer of cells (a-k) or may be many-layered 

 (i) ; the former are termed non-stratified, the latter stratified, 



epithelia. The cells of an epithe- 

 lium may be flattened (c, e), their 

 edges being cemented together so 

 as to form a continuous mem- 

 brane ; or they may be cubical 

 or cylindrical or prismatic (a, b) ; 

 in the case of a stratified epithe- 

 lium the cells may be of different 

 forms in different strata (i). The 

 epidermis, which covers the outer 

 surface of the body of an animal, 

 is an example of an epithelium ; 

 sometimes it is stratified, some- 

 times unstratified ; its cells some- 

 times possess cilia, sometimes are 

 devoid of them. Lining the 

 internal cavities of the body are 

 layers of cells, or epithelia, some- 

 times in a single layer, sometimes 

 in several layers, sometimes 

 ciliated, sometimes non-ciliated. 



Glands (Fig. 12) are formed 

 for the most part by the modifica- 

 tion of certain cells of epithelia. 

 In many cases a single cell of the 

 epithelium forms a gland, which 

 is then termed a unicellular gland 



(Fig. 12, A). The secretion (or substance which it is the function of 

 the gland to form and collect) gathers in such a case in the interior 

 of the cell, and reaches the surface of the epithelium through a 

 narrow prolongation of the cell which serves as the duct of the 

 gland (B). In other cases the gland is multicellular ioimed of a 

 number of cells of the epithelium lining a depression or infolding, 

 simple or complex in form, of the latter (D G). In the central 

 cavity of such a gland the secretion ^collects to reach the general 

 surface or cavity lined by the epithelium through the passage or duct. 



FIG. 12. Diagram to illustrate the struc- 

 ture of glands. A, unicellular glands in an 

 epithelium ; B, unicellular glands lying 

 below epithelium and communicating 

 with the surface by narrow processes 

 (ducts) ; (7, group of gland-cells ; D, 

 group of gland-cells lining a depression ; 

 E and F, simple multicellular gland ; 

 O, branched multicellular gland. (From 

 Lang.) 



