90 PROTOPLASM OF PROTOZOA 



flagella, axial filaments, cilia, myonemes, and supporting fibers (mor- 

 phonemes) of various kinds. The fact that these are elastic is too well, 

 known to need more than mention here. Spindle fibers, on the other 

 hand, are often thought to be artifacts, but Cleveland (1935), work- 

 ing on the hypermastigote flagellates, has pulled the centriole out of 

 position; the chromosomes were also displaced, but both centriole and 

 chromosomes immediately sprang back into position when released. This 

 argues not only for the reality of the chromosomal and spindle fibers, but 

 also that they are structures of considerable elasticity. If one considers 

 the aphorism of Needham (1936) "that biology is largely the study of 

 fibers," these fibrillar structures of the Protozoa are of great interest 

 because they consist of parallel aggregations of the submicroscopic elon- 

 gated particles (micelles) of protoplasm (see Taylor, infra, Chapter IV) . 



CONTRACTILITY 



We may distinguish between active contraction, as in muscle fibers, 

 and elastic shortening after having been stretched, as in elastic fibers in 

 the higher animals. Although there are many examples of contractility 

 and elasticity, in the Protozoa, associated with differentiated myonemes or 

 morphonemes respectively, there are also many instances of active con- 

 traction in the absence of any optically differentiated structure. Accord- 

 ing to Lewis (1926), theories of contractility must be based on the 

 presence of a contractile molecule, because the fibrillae seen in heart 

 muscle in tissue culture are not "true" cytological structures but are 

 due to reversible gelation. Faure-Fremiet (1930) also holds that the 

 gelified condition is often bound up with the existence of internal 

 fibrillar structures, which disappear when solution occurs. That such 

 fibrillae appear and disappear may, of course, be caused by aggregation 

 and disaggregation of smaller invisible fibrillae, or may even be due to 

 changes of refractive index. It is well known that objects that are of the 

 same refractive index, transparency, and color cannot be seen, even in 

 the dark field (Schmidt, 1929). 



It is generally assumed that contraction of the gelled ectoplasmic 

 cylinder in Atnoeba forces the more fluid endoplasm forward (Schaeffer, 

 1920; Pantin, 1923; Mast, 1926b). The contraction is thought to be 

 caused by the fact that the gel-sol process at the posterior end of the 



