40 



COMPARATIVE ANATOMY 



CHAP. 



fluous or waste material, which they then remove out of the body. 

 Dermal glands secrete mucus or other substances. 



When glandular cells remain single they form unicellular glands ; 

 if many combine for the same purpose they are called multieellular 

 glands. 



In the simplest cases, these glands are epithelial cells, chiefly 

 distinguished by their size, and often pear-shaped. The nucleus 



lies at the basal end of the cell, i.e. 

 the end away from the free surface 

 of the epithelium. The secretion 

 collects in the cell and is pushed 

 forward towards its surface. Where 

 a cuticle is developed it is often 

 broken through above the gland cell, 

 forming a pore for the discharge of 

 the secretion (Fig. 35, A}. 



Unicellular glands often partially 

 leave the epithelium, the greater part 

 of the gland projecting into the 

 underlying tissue. They then only 

 penetrate between the epithelial cells 

 by means of thin processes, the 

 efferent ducts (Fig. 35, _'). 



A greater or smaller area of 

 epithelium often consists entirely or 

 principally of glandular cells (Fig. 

 35, C). Such areas generally sink into 

 the epithelium, so that a glandular 

 epithelial sae arises, into whose cavity the secretion is emptied (Fig. 

 35, D, E}. Such sacs again may form many branches (Fig. 35, G). 

 The glandular portion is then frequently limited to the blind and 

 often lobate ends of the sacs, while the rest serves as an efferent duct. 

 Glandular cells again can sink under the epithelium of these glandular 

 sacs, and only remain connected with it by their efferent ducts (Fig. 

 35, F). 



It is only natural that certain cells of the epithelium which 

 envelopes the body should undertake the function of carrying on the 

 relations between the body and the outer world, i.e. of receiving im- 

 pressions and imparting them to the body. Such epithelial cells are 

 called sensory cells. They can either occur singly or form, in com- 

 bination, a sensory epithelium. Originally these cells are very slightly 

 differentiated, very slightly qualified for the reception of varied 

 impressions. But here "also, as the organism grows more perfect, 

 division of labour steps in. Certain sensory cells appear qualified for 

 the perception of definite sorts of impressions only. They enter into 

 combination with certain tissue elements and form specific sensory 

 organs, which give rise to perceptions of either touch or sound, sight, 



G 



FIG. 35. A-G, Various forms of glands. 



