94 



GENERAL PRINCIPLES OF ZOOLOGY. 



tubular glands belong the liver, kidney and sweat glands 

 of man ; to the acinous belong the salivary glands, not 

 only of the vertebrates, but also of the arthropods and 

 mollusks. 



Sexual Epithelium. In connection with the glandular 

 epithelium we consider appropriately the sexual cells. As 



, 



f 







FIG. 29. Acinous salivary gland of Orthezia cataphrai ta (after List) ; in some acini the 

 nuclei and boundaries of the cells are shown. 



the secretion of the former must be expelled from the body, 

 so also the sexual cells form elements which are no longer 



FIG. 30. Germinal epithelium of a medusi. ek, ectoderm ; en, entoderm ; a, egg; 



e, epithelium. 



a part of the organism, and must reach the exterior in order 

 to perform their function. Just as the gland-cells are 

 usually scattered among ordinary epithelial cells, so the 

 sexual cells, almost without exception, lie embedded in 

 epithelium ; it may be in the epithelium of the skin (Fig. 30) 

 of the gut, of the body-cavity, or of parts separated from 

 this (Fig. 31). This connection of the sexual cells with 



