PLASMAGENES 389 



1949, Spiegelman 1948, Medawar 1947, Ephrussi 1953 ; a somewhat more 

 reserved attitude is taken by Brachet 19526, Beadle 1949, Waddington 

 i94Sb, Sonneborn 1951a, &, and Haldane 1954. General reviews of the 

 phenomena, less concerned with development, are Lederberg 1952, Caspari 

 1948, Cold Spring Harbor Symposia Nos. 11 and 16, 1948 and 1951, and 

 the symposium pubhshed as Unites hiologiques donees de continuite genetique 

 (Paris 1949). 



etc. 



Figure 18.1 



A self-reproducing cyclic system. It is supposed that when A is' added to the 

 cell, it induces the formation of enzyme a, which converts it, together 

 with m from the rest of the cell, into B. After several such steps, during 

 which raw materials (e.g. n) are used, and other substances (e.g. p, q) pro- 

 duced, the system finally absorbs energy (Z) and results in the formation of 

 more A than was originally supplied. This restarts the original cycle, and 

 also brings into being a new cycle. (After Pollock 1953.) 



From the point of view of their possible importance in differentiation, 

 plasmagenes may be considered under the following headings (c£ Fig. 

 18.2). 



I. Exogenons plasmagenes 



Many viruses, such as those producing disease, are clearly not essential 

 constituents of the animal or plant and are introduced into the cell from 

 outside. There is considerable variation in the ease with which this intro- 

 duction can take place. Some of the bodies which were orginally thought 

 of as true plasmagenes should be regarded as essentially exogenous factors 

 for which infection is rather difficult. This probably appHes to the kappa 

 particles in Paramecium (Sonneborn 1947, 195 1<?, b). In these Protozoa, 



