KERATIN AND MOLECULAR BIOLOGY 



39 



lipid), react with the fixatives (osmium tetroxide or potassium per- 

 manganate) more vigorously than the inner layer, L, and thus have 

 become " stained." 



In some simple organisms the cell membrane is protected by a 

 secretion of slimes or mucins (Fig. 14(b)); in others this takes a more 

 definite form as an external pellicle (Fig. 14 (g)). In these secretions it is 

 perhaps permissible to trace the primitive forerunners of both the adhesive 

 intercellular cements and of the elaborate extracellular coats found in 

 some higher animals. According to Weiss (1933) cells freed from tissues 

 and cultivated in vitro surround themselves with a colloidal exudate. 



Cilia, flagella, etc. Cilia and flagella are very similar in internal structure 

 although somewhat different in behaviour. They are long motile pro- 

 trusions of the membrane and in electron micrographs reveal a complicated 



ft rt (TS fj 



J U U 111 •;,...;•• L 



(0 



m&tm 



Id) 



(e) 



(f) 



Fig. 19. Surface membrane specializations. See text. 



internal structure (Fig. 19 (a)). (Bradfield, 1955; Fawcett and Porter, 

 1954; and Manton, 1952). In cross-section they are seen to consist 

 of the enveloping plasma membrane, nine pairs of peripheral 

 filaments, a central pair of filaments of a different character from the 

 nine and an amorphous ground substance. The peripheral filaments in 

 the protozoa are seen to end on a basal body within the cytoplasm which 

 consists of a small short cylinder apparently consisting of nine short 

 rodlets and which may control the organization of the bundle of filaments. 

 The centriole found in many cells of animals appears to consist of an 

 identical object and here too it may assist in organizing cytoplasmic 

 proteins (Porter, 1957; Grimstone, 1961). 



