Cell Constitution 



49 



bands of varying density, thickness and af- 

 finity for stains like plaosphotungstic acid. 

 This is true for vertebrate muscle, paramyo- 

 sin and collagen. Detailed intraperiod struc- 

 ture, in the form of fine bands having char- 

 acteristic position and density, has been ob- 

 served in several fibrous proteins. The com- 

 bination of x-ray and EM data may provide 

 clues as to the physical and chemical mean- 

 ing of such accurately repeating axial struc- 

 ture. In some cases it is suspected that a 

 globvilar component may be associated with 

 the fibrous component in forming the pe- 

 riodic structure and in determining the 

 properties of the fiber. 



Cilia, Flagella and Sperm Tails. It has long 

 been suspected, on the basis of their positive 

 form birefringence (Schmidt, '37), that these 

 microscopically visible fibrous structures are 

 composed of still finer submicroscopic fibrous 

 components. EM examination has demon- 

 strated that they are, in fact, bvindles of 

 fibrils, each fibril being 300 to 600 A in 

 width and running the full length of the 

 cilium, flagelKim or sperm tail. 



A very interesting point is that the num- 

 ber of these fibrils is relatively constant. In 

 mammalian sperm tails there are usually 

 eleven fibrils, of which two may be thinner 

 than the remainder. According to Fawcett 

 and Porter ('52), molluscan cilia also con- 

 tain eleven fibrils, two in the center sur- 

 rovmded by nine peripheral filaments. The 

 detailed mechanism of the formation of these 

 fibrils, presumably in association with the 

 centriolar apparatus, constitutes a challeng- 

 ing subject for future EM study. 



There is some indication of an axial re- 

 peating pattern in such fibrils (Grigg and 

 Hodge, '49; Fawcett and Porter, '52) but 

 thus far no clear-cut proof of such a period 

 has been demonstrated. 



Around the sperm tail may be distin- 

 guished a sheath composed of one or more 

 helically coiled fibrils. For a good descrip- 

 tion of the fine structure of plant cilia the 

 reader is referred to Manton ('52). 



The structural and chemical basis of the 

 contraction of cilia, flagella and sperm tails 

 has not yet been demonstrated. Unlike mus- 

 cle, the contraction is not a longitudinal 

 shortening and thickening of the fibers but 

 rather a screw or helical type of beating. It 

 is not clear whether the filaments are them- 

 selves contractile or whether they provide 

 chiefly mechanical rigidity as was supposed 

 in older theories. 



The flagella of motile bacteria appear to 

 be individual fibrils abovxt 120 A wide pro- 



truding from the bacteria rather than bun- 

 dles of fibrils aggregated to form individual 

 flagella. The flagella may be isolated in rela- 

 tively pure form and their composition and 

 strvicture studied. According to Astbury and 

 Weibull ('49) they give the alpha wide- 

 angle diffraction pattern characteristic of the 

 KMEF class. An axial period of about 400 A, 

 similar in magnitude at least to that of ver- 

 tebrate muscle, has also been found by Ast- 

 bury (personal communication), who sug- 

 gests that such flagella may be considered 

 "monomolecular hairs or muscles" (Astbury, 

 '51). 



Trichocysts. The Paramecium trichocyst is 

 an elongate, thin-walled tube terminating in 

 a dense, pointed tip. The wall as well as the 

 tip shows birefringence positive with respect 

 to the axis of the trichocyst (Schmidt, '39). 

 In the EM the wall of the tube shows cross- 

 striations with an axial period of 550 A (pos- 

 sibly the main period is four times this) and 

 intraperiod fine structure (Jakus, '47). It 

 would appear that the trichocyst wall is 

 composed of a fibrous protein or conjugated 

 protein of unknown nature, the fiber axis 

 being parallel to that of the trichocyst shaft. 

 It will be interesting from the comparative 

 biostructvire view to discover to which class 

 of proteins, in the x-ray diffractionist's terms, 

 this material belongs. 



CYTOPLASMIC SUBMICROSCOPIC 

 FIBROUS STRUCTURES 



The view that cytoplasm contains, besides 

 the fibers visible in the microscope, a sub- 

 microscopic fibrous network or lattice rests 

 not only on the tendency for fixatives to pro- 

 dvice fibers but also on a number of physical 

 properties manifested by unfixed protoplasm. 

 When protoplasm is made to flow through a 

 capillary tube the flow is non-Newtonian in 

 nature, e.g., the rate of flow is not propor- 

 tional to the force applied (Pfeiffer, '40). 

 This is a property of solutions containins* 

 elongate, threadlike, rather than spherical, 

 particles. When frog eggs were centrifuged 

 in a centrifvige microscope equipped for ob- 

 servation in polarized light, it was found 

 that birefringence was developed which is 

 positive with respect to the direction of the 

 centrifugal field (Pfeiffer, '42). This indi- 

 cates the existence in the cytoplasm of elon- 

 gate, fibrous particles which were aligned by 

 the centrifugal force. The viscosity of cyto- 

 plasm can be estimated by a variety of 

 methods (Heilbrunn, '52). Although it is 

 possible to do this only semiquantitatively, 



