ZOOFLAGELLATES 139 



this adhesion is not demonstrated. The flagellum in all forms 

 em ^es from the cell at the base of a reservoir that is slenderer 

 tiStn, but otherwise like, that of euglenoids (Clark, 1959); a 

 contractile vacuole empties into the reservoir. 



The kinetosome typically projects for about half its length into 

 the cavity of the reservoir ; it is capped by a transverse diaphragm 

 above which the central fibrils of the flagellar axis arise. Basally, 

 the nine fibrils of the kinetosome end, in all forms studied, close 

 to the kinetoplast; opinion differs as to whether or not actual 

 contact with or even penetration of the kinetoplast membrane is 

 demonstrable. The kinetoplast is a disc-shaped to ovoid body, 

 usually situated with its long axis perpendicular to the kinetosome 

 ot slightly cupped below it. It is distinctly Feulgen-positive and 

 has been shown by Steinert, Firket, and Steinert (1958) to 

 ncorporate tritiated thymidine. In addition, it reacts positively 

 to mitochondrial stains. The function of this extranuclear, DNA- 

 containing organelle is totally unknown in spite of ample and 

 active interest in the subject. Natural occurrence of individuals 

 lacking the kinetoplast has been observed, and akinetoplastic 

 strains of some mammalian parasites may be maintained 

 indefinitely in vivo, but not in vitro (Steinert, 1960). In the past, 

 the kinetoplast has often been confused with a flagellar basal body 

 or blepharoplast and with a parabasal body. It most definitely 

 does not resemble either of these bodies morphologically, hence 

 such misconceptions are no longer justifiable. 



In its ultrastructure the kinetoplast is like no other familiar 

 organelle (see Figs. 50, PL XIII; 5, PL I). It contains a reticulum 

 of filaments showing a strong antero-posterior orientation, 

 typically surrounded by a zone of varying density and limited 

 externally by two unit membranes. The peripheral zone may be 

 narrow or may occupy the bulk of a spheroidal kinetoplast, with 

 the reticular structure constituting a plate or band across the 

 middle. Membranous structures resembling mitochondrial cristae 

 are seen in this peripheral region in most trypanosomes (Clark and 

 Wallace, 1960), and Steinert (1960) was able to show in Trypano- 

 soma mega a distinct mitochondrion completely continuous at one 

 end with the kinetoplast. Small mitochondria also are scattered 

 elsewhere in the cell. In all micrographs that resolve these clearly, 

 their internal membranes have the form of cristae, irregular in 



