130 KERATIN AND KERATINIZATION 



arranged into parallel sets of fibrils forming approximately a right angle 

 with a similar set above and below it (Weiss and Ferris, 1954). The 

 ultimate orienting influence is clearly the surface of the animal, which 

 ensures that the whole formation lies parallel to it; the immediate con- 

 trolling influence is not so obvious (Plate 14B). In other instances the 

 geometry is even more complex and sometimes of surprising regularity. 



Such sheets may be formed by exfoliation from a surface composed of 

 the cells that secrete the precursor. Here we can conceive of two kinds of 

 organizer: (a) a pattern or " die " on the cell surface or (b) the existing 

 pattern of the preceding sheet, which acts as a template for the assembly 

 of its successor. The peritrophic membrane lining the mid-gut of insects 

 could be an example of the first suggestion (Mercer and Day, 1952). Here 

 the secreting cells are covered with projecting microvilli (the brush border 

 of histology) with a cross-sectional diameter about the same size as the 

 holes in the membrane. We could imagine the filaments forming in the 

 grooves between the studs on the surface. Sections of the cell surface, 

 although showing the pattern of microvilli, and the layers of shed mem- 

 brane, have not yet provided an example of a membrane in the act of 

 formation; therefore decisive evidence is still wanting. 



The collagen meshworks in skin, and in earthworms (Rudall and Reed, 

 1948) seem to form some distance from the cell surfaces, which are 

 covered with amorphous material. The " self-template " seems more 

 likely here. We can form a conception of how this could operate by 

 supposing that the upper surfaces of fibrils have " studs " on them which 

 fit into " holes " in other fibrils when these are laid across then at right 

 angles. 



This discussion of fibrogenesis and organization has been limited to the 

 special case in which the elementary fibrous unit is a fine filament (or 

 ribbon). While this is applicable to many systems, we must expect that 

 other devices will be found such as Rudall has described in the secretion 

 of the colleterial gland of a mantid (p. 126). 



Tactoids, familiar from their occurrence in tobacco mosaic virus (TM V) 

 solutions, have been proposed as fibre-forming elements (Bernal, 1940). 

 The rodlets of TMV are not unlike the protofibrils under consideration 

 here and it seems quite probable that, in some instances, e.g. the bundles 

 of fine filaments of keratin in hair cells, the same forces which maintain 

 tactoids are operating. Spherulites and sheaves, e.g. in F-insulin seem to 

 result from growth by aggregation from a single, or a group of centres in 

 the absence of external orienting forces. The filaments must also be 

 supposed to possess little lateral attraction. 



As mentioned in Chapter I calcium salts may be deposited in associ- 

 ation with fibrils of collagen to form bone. The deposition seems to be 

 initiated at definite sites in the 640 A banded collagen fibril and the earliest 



