CELLULAR DIFFERENTIATION S3 



Membranes bounding the body cavity are called serous membranes (endothe- 

 lium). The lining of the digestive tract is described as a mucous membrane. 



Epithelial cells often secrete upon their outer surface a layer of material 

 {cuticula), which serves to protect the cells beneath and the organism as a whole 

 from external influences (as the covering of the cray-fish). Prom the epithelium 

 arise various outgrowths, as scales, hair, feathers, and the Uke. 



70. Glandular Epithelium. — The ordinary columnar or pavement epithelium 

 may here and there present cells or areas of cells which are specially active in 

 producing and pouring out on their free surface certain materials, called secretions. 

 In its simplest form the gland or secreting surface may consist of a single cell, as 

 the goblet or slime cells (Fig. 17, a). 



Such a cell may become much enlarged and sink below the general level of the 

 epithelium, retaining in the meantime a narrow connection with the exterior (Fig. 

 17, b). Multicellular glands represent areas of such cells which have sunk below 



Fig. 21. 



o 



Fig. 21. Section througK ovary of a young Mammal (modified from Wiedersheim). The eggs 

 (o) are seen to be formed from the epithelium by a process somewhat more complex than in Fig. x8. 

 e, connective tissue of ovary; e, epithelium; /. follicle of epithelial cells in which the ova ripen; o. ova 

 in different stages of ripeness. 



Questions on the figure. — In the ovary of the mammal what additional service 

 does the epithelial layer render the ovum after its formation? Is it apparent that 

 there is anything gained by the sinking of the ovarian follicles into the tissue of the 

 ovary, instead of escaping immediately, as in Fig. 20? 



the surrounding surface, forming a tube- or flask-shaped cavity, which may become 

 very much branched. Glands with such branched ducts are described as compound. 

 They consist of numerous final secretory sacs communicating by ductules with a 

 common duct or outlet to the surface. Transitional conditions between the 

 simple secretory epithelium and the compound gland may be seen in Fig. 18. 



71. Sensory Epithelium. — In the lower animals there may be found here and 

 there over, the surface of the body modified epithelial cells, which are specially 

 capable of being stimulated by contact or other stimuli to which the organism may 

 be exposed. ' Likewise in higher forms we find highly specialized areas of sensitive 



