GLANDS 



511 



d.: 



cells of the gonads which are mesodermal in origin, are not 

 epithelial. 



Non-glandular simple epithelium is found in the lungs and in 

 various ducts. In the terminal bronchioles, the final air-tubes 

 which lead into the lungs, it is cubical, while in the alveoli or 

 small blind chambers which 

 make up the mass of the lung 

 itself it is pavement. A simple 

 columnar ciliated epithelium 

 lines the bronchi. 



The part of the coelomic wall 

 which gives rise to the germ cells 

 is often called germinal epi- 

 thelium, but if ' epithelium ' is 

 considered, as it should be, as a 

 name for a specialised tissue 

 made from specialised cells, it is 

 not appropriate to a part of the 

 body consisting of totipotent 

 cells, that is, cells which can 

 develop into a complete organism 

 and give rise by mitosis to epi- 

 theliocytes, mechanocytes, and 

 amoebocytes in all their variety. 

 Sensory epithelium, which is best 

 regarded as a specialisation of 

 nervous tissue, is mentioned 

 below (p. 534). 



NERVOUS TISSUE 



Fig. 398. — Diagram of part of a 

 neurone highly magnified. 



ax.. Axon ; c.b., cell body ; d., dendrites : 

 nu., nucleus. 



Nervous tissue is derived from 

 an undifferentiated epithelium 

 of the embryo, but its cells are so different from all others that 

 it is best treated on its own. The typical nerve cell or neuron 

 (Fig. 398) is large— of the order of 100 /x in diameter— with a 

 conspicuous nucleus and a number of branched processes called 

 dendrons or dendrites. There is also usually another process, 

 much longer and seldom branched except at its extreme end, 

 called an axon. The dendrons, and often the branched ends, 

 called terminal arborisations or telodendria, of the axons, come 



