12 STRUCTURE 



basal body seems to be consistent, appearing in sections as an 

 empty cylinder whose walls contain continuations of the nine 

 peripheral fibrils of the shaft, while the two central fibrils of the 

 ciliary shaft end at about the level of the cell surface. In many 

 cases fibrillar structures are attached to these basal bodies and may 

 be connected to other structures of various sorts including the 

 basal bodies of other cilia. 



This basic pattern is not entirely consistent, however, for 

 important diflFerences occur. The structural pattern of cilia 

 remains fairly constant, functional differences depending on their 

 co-ordinated action and sometimes on their association into 

 compound structures. The flagellar pattern is more widely 

 modified in protozoan and algal flagella as well as in sperm tails. 



The most recent electron microscope work with improved 

 techniques indicates that the basic pattern is even more complicated 

 than was believed a very few years ago. These reports are so 

 consistent in some details that it seems reasonable to base a 

 description of " a typical cilium " upon a few of the more recent 

 observations and follow this with an account of the variations from 

 this basic pattern. 



It should not be imagined that we have reached the limits of 

 magnification of biological material with the electron microscope. 

 Theoretically the electron microscope can be used to examine 

 structures down to about 1 A, and instruments which can resolve 

 objects smaller than 10 A are in use, but the limit for good resolu- 

 tion in biological material is at present in the region of 30 A, or 

 sometimes 20 A, although few workers have achieved this. As 

 methods of fixation, ultra-thin sectioning and " electron staining " 

 improve, it should be possible to approach more nearly to the 

 theoretical limit. We can therefore expect the electron microscope 

 to reveal yet more of the details of cilium structure as techniques 

 are refined. 



The structure and functioning of cilia and flagella are so 

 similar that one description will hold good for both organelles in 

 most respects. A term descriptive of both cilia and flagella is really 

 required, but, since none is in common usage, the organelle will be 

 referred to as a cilium in general descriptions, and the term 

 flagellum will be used only for structures which are believed to 

 have a flagellar beat. 



