216 ELECTRON-MICROSCOPIC STRUCTURE OF PROTOZOA 



streams of cytoplasm seems mechanically implausible, and the 

 electron-microscope picture needs further investigation. All 

 theories of ameboid movement require either a fairly free slippage 

 of the plasmalemma over the ectoplasm or a rapid interconversion 

 of membrane and superficial ectoplasm. The former seems more 

 likely, although in a great many protozoa the probability that 

 membranes can be synthesized very rapidly finds strong support 

 in electron-microscope data. 



Amebae and most flagellates appear to be limited by a single 

 unit membrane. For the sporozoa, euglenoids, and ciliates, two 

 or more membranes are commonly described. There is something 

 conceptually disturbing about the idea of two limiting membranes. 

 What does the outer one limit ? Or if, as is usually reported, the 

 outer is the only one that is continuous over cilia or potential 

 orifices, then what becomes of the inner one here ? A permanently 

 discontinuous membrane is hard to reconcile with the usual 

 picture of membrane formation and function; cells and mem- 

 branous organelles generally consist of closed sacs, vesicles, and 

 tubes. If two true membranes are present, one would expect that 

 something is present between them that must be limited on both 

 sides, and that both membranes are continuous, defining a 

 completely enclosed peripheral space. This may be true for 

 Plasmodium, for example, but the situation in euglenoids and many 

 ciliates remains enigmatic. 



For at least some ciliates, the presence of three superficial 

 membranes is established, the two inner ones composing the 

 mosaic of enclosed alveoli that define the silverline pattern. This 

 has now been demonstrated in Paramecium, in three tetrahymenids, 

 and in one astome, and the question arises whether a similar 

 system exists in other ciliates. Two difficulties prohibit an easy 

 answer. In the first place, the silverline system is not identifiable 

 because different methods of silver impregnation may affect 

 different morphological entities. The commonest (Klein, Gelei, 

 and Chatton-Lwoff) techniques reveal kinetosomes and superficial 

 linear patterns in most, but not all, ciliates (as well as in many 

 flagellates, sporozoa, etc.) ; other methods may impregnate internal 

 structures. In the second place, most electron microscopists have 

 failed to observe any structures corresponding to the silverline 

 patterns described for their subjects. Precise discrimination in 



