78 



GENERAL PRINCIPLES OF ZOOLOGY. 



acinous belong the salivary glands, not only of the vertebrates, but 

 also of the arthropods and molluscs. 



FIG. 30. Tubular glands. (After Toldt.) A, glands of Lieberkuhn from the human 

 intestine; A', of the conjunctiva of the eye; B, of the cat's stomach; C, from the 

 medullary pyramids of the dog's kidney; U, from the cortex of the rabbit's 

 kidney. 



Sexual Epithelium. The sexual cells may be considered in 

 connexion with glandular epithelium. As the secretion of some 



FIG. 31. Acinous salivary gland of the aphid Orttiezia cataphracta. (After List.) In 

 some acini the nuclei and boundaries of the cells are shown. 



glands must be expelled from the body, so the sexual cells are 

 elements which differ from the rest of the organism, and must 

 reach the exterior in order to perform their function. Just as the 



