CONCLUSIONS 223 



present to be organized ; or perhaps they are made from the same 

 original materials, but asymmetric physical or chemical fields in 

 the surrounding cytoplasm determine how they will be linked 

 into fibrils and which direction they will take. The problem is so 

 remote from present understanding that more speculation seems 

 useless. 



Given a plethora of fibrillar designs and constructions, we can 

 scarcely conceive of enough functions to go around. A few come 

 obviously to mind : contraction, mechanical support, information 

 transfer. 



As previously mentioned in this review, currently respected 

 hypotheses of the mechanism of protoplasmic contraction require 

 that this occur either by the folding of protein chains or else by 

 the sliding of interdigitating filaments (actin and myosin in the 

 case of striated muscle) within a closely packed bundle. Contrac- 

 tion by folding of chains in a gel network is adduced to explain 

 ameboid locomotion according to more classical theory, but both 

 mechanisms or something like the second one alone could operate 

 in ameboid and foraminiferan movement according to recently 

 proposed views. 



Morphologically identified fibrous structures that certainly 

 possess the property of contractility are flagella and cilia, which 

 indubitably beat, and the myonemes of vorticellid stalks, which 

 unquestionably shorten. We have nothing here to add to the 

 discussion of the former in Chapter 2. The myoneme of the 

 vorticellid stalk consists of a long bundle of roughly parallel fine 

 filaments of indeterminate length, penetrated and incompletely 

 surrounded by membranous canaliculi. Its general similarity to 

 smooth muscle (which has been relatively neglected by biophysi- 

 cists) has been repeatedly pointed out. 



If the filamentous vorticellid myoneme be accepted as a truly 

 contractile organelle, then it becomes reasonable to assume that 

 similar constructions also have this capacity. These include the 

 endoplasmic body myonemes of peritrichs, the M-bands oiStentor, 

 and the axes of prehensile tentacles in Ephelota. Of the other 

 structures in class 3 above, the central filament bundle in Actino- 

 sphaerium axopodia; the circular myonemes of gregarines; the 

 layered, filamentous, ecto-endoplasmic boundary in the astomes 

 and hotricha; and the interconnected, pellicular and digestive-sac 



