FINE STRUCTURE 59 



vesicles terminate on the inner membrane. Both membrane and 

 vesicle w^alls showed small opaque particles and the interstices 

 gave the appearance of a finely particulate matrix. Empty mito- 

 chondria were found, as well as others without bounding mem- 

 branes amongst the normal forms of these bodies, reminding one 

 of Weisz's (1949a) suggestion that the mitochondria are utilized 

 in starvation and regeneration. 



Food reserves are also present in the form of fat droplets and 

 glycogenoid granules. In addition, multiformis and introversus have 

 yellow, brightly refringent bodies or crystals in the endoplasm, 

 the nature of which has not yet been determined. 



Altogether, this detail of fine structure represents about as 

 extensive and intensive a cytodifferentiation as we are likely to 

 find, comparing favorably with that of the most complex hypermas- 

 tigont flagellates in arthropods and ruminant commensal ciliates. 

 The number of definable parts which have been "compacted** 

 into the minute volume of a stentor is quite amazing and attests the 

 extremely fine-grain structure possible to organisms. For instance, 

 there are about 32,500 fibers in the complement of ribbon bundles 

 or ^m-bands alone, not to mention the countless cortical granules, 

 etc. Not only in number but in their greater diversity the minute 

 parts of an organism like Stentor stand in contrast to the cyto- 

 differentiation of most tissue cells. This difference in manifest 

 complexity is one of the reasons why some biologists have hesitated 

 if not refused to call protozoa cells or unicellular forms. Yet a 

 stentor represents no more or less a separate nucleocytoplasmic 

 system than a neurone. And a bridge between protozoan and tissue 

 cell may perhaps be found in the egg; for if one refuses to call a 

 fertilized egg a cell, all seem to agree that its cleavage products are 

 cells and from either of the first two cells, let us say, the whole 

 complex multicellular organism can be derived by embryogenesis. 

 It may therefore not be too much to infer that such a cell is 

 intrinsically as complex as Stentor, if not more so, but manifesting 

 this complexity in development through multicellularity instead of 

 more directly in itself. 



What, then, shall we *'do" with all the complex cytodifferentia- 

 tion we find in Stentor ? One approach is to study, if possible, 

 certain types of parts in themselves. For example, the ^m-bands 



