PROTOZOA AS CELLS 47 



fibrils of the flagella are present in the cortex of the syncytium 

 although the flagellar membranes have disappeared and no 

 flagella are evident externally. Ehret and Powers (1959) argue 

 that kinetosomes of gullet organelles of 'Paramecium arise de novo, 

 physically separated from any preexisting ones, but their evidence 

 on this score is derived from light microscope studies, which have 

 led other workers to opposite conclusions, so the question must 

 be regarded as unsettled. 



For metazoan cells, the problem of centriole replication has 

 been considered recently by Bernhard and de Harven (1960) and 

 by Gall (1961). According to the former authors, scrutiny of 

 centrioles in a variety of tissues often reveals the presence of 

 "satellite" bodies, small dense structures about 70 m/x in diameter. 

 Several of these bodies may appear anywhere within a zone of 

 about 250 m/x or so around the cylindrical centrioles, and often 

 some of them are united by slender dense bridges to the centriole 

 proper. A single fortunate section of a cell from a leukemic mouse 

 liver showed the two centrioles lined up one behind the other. An 

 arrangement of fibrous structures at their adjoining ends distinctly 

 suggested two additional, very short cylinders oriented at right 

 angles to the mature centrioles. Bernhard and de Harven propose 

 that these are daughter centrioles formed by a process of lateral 

 budding from the mothers, and that the satellite bodies represent 

 some sort of precursors. Gall's (1961) important paper likewise 

 illustrates the appearance of short "procentrioles" at right angles 

 to the parent centriole. In atypical spermatocytes of the snail 

 Viviparus, multiple procentrioles — as many as eight in a single 

 section — may be seen, lined up radially about one end of a mature 

 centriole (these ultimately all mature and produce flagella). The 

 often observed arrangement of two centrioles at right angles to 

 one another thus may be explained as the persistence of a relation- 

 ship established when a new centrioje is formed. Later, the two 

 centrioles may separate; an alternative position commonly 

 observed is in single file, with a common long axis. This was 

 true of the "budding" mother centrioles in the figure by Bernhard 

 and de Harven and often obtains when one centriole in metazoan 

 tissues is in the process of forming a cilium. Indeed, Bernhard 

 and de Harven observed centrioles in this position, from one of 

 which an abortive cilium was projecting into a depression of the 



