CELL TYPES 



505 



very doubtful whether one can ever change into another. Epi- 

 theUocytes are flattened, and their most characteristic property 

 is that they stick to each other or to anything else, such as a 

 glass surface, so that they form sheets or membranes and are 

 rarely seen alone. When their growth is slowed down they often 

 produce the protein keratin. Mechanocytes (often called fibro- 

 blasts, although this word is best used in a more restricted sense) 



Mechanocytes 



Epitheliocytes 



Amoebocytes 



Epitheliocytes of 

 different form 



Fig. 393. — The primary groups of cells as seen in tissue cultures. The epithelio- 

 cytes in b are probably secondarily derived from mechanocytes.— From 

 Willmer. Bourne, Cytology and Cell Physiology, 2nd edition, 195 1. Clarendon 

 Press, Oxford. 



are elongated and are capable of a mysterious type of gliding 

 movement, but they usually form a three-dimensional network. 

 Amoebocytes are, as their name implies, amoeboid in form ; 

 they remain isolated from one another, and move relatively 

 rapidly by the formation of blunt pseudopodia, which are also 

 used for ingesting foreign particles. The three types of cell differ 

 also in their nutritive requirements ; epitheliocytes are especially 

 dependent on an adequate glucose concentration in the medium, 

 mechanocytes on unknown substances present in extract of 

 embryos, while amcebocytes are the least exacting. 

 M.z. — 17 



