14 Introduction to Animal Morphology. 



may be elongated, fibre-like (as in the crystalline lens), 

 branched, ribbed, pigment-holding, or goblet-shaped, 

 as in the mouth of a fish or reptile, the throat of a frog, 

 or in the vertebrate small intestine (except in birds). 

 This variety has an open mouth (stoma) of naked pro- 

 toplasm, as have some of the ordinary cylinder cells 

 in the stomach of man [Schultze)^ or the mouth may 

 be partially closed by parallel rod-like particles [Bret- 

 tauer and Steinach). The body of such a cell (theca) is 

 dilated and then narrows into a pedicle. These cells, 

 as having the thinnest coating, are supposed to be the 

 most active agents in intestinal absorption (denied by 

 Th. Eimer ; they are described by Arnstein and 

 others, as only altered cylinder ciliated cells).* 



In homoplastic animals, the fluid contents of each 

 cell suffice for the physiological requirements of the 

 animal, but growing complexity requires functional 

 division of labour, and some epithelial cells are 

 clustered together for purposes of secretion. These 

 may be on the surface or in a shallow or deep, simple 

 or branched, recess : such a structure is called a gland. 

 The process of secretion may be one of the liquefaction 

 and conversion of the cell contents into the material 

 to be cast off", as in mucous glands, in which on sec- 

 tion a distinct layer of metamorphosing cells may be 

 seen with the microscope (Lunula of Gianuzzi). In 

 other cases there is not the direct cell waste. 



The second group of vegetative tissuesf includes 



* The cells of the middle layer of the epithelium in the bladder of some 

 Mammals are tailed, and their processes seem joined to papillary emi- 

 nence of the connective tissue underlying. 



t His describes these as parablastic and developed secondarily in the 

 embryo. 



