396 PRINCIPLES OF EMBRYOLOGY 



regeneration when provided with an anterior node. Further, if all but the 

 posterior node is removed from an intact individual, the mouth and other 

 anterior parts degenerate and disappear. 



There is, then, a gradual loss by the posterior nodes of 'potency' to 

 mediate regeneration or to support differentiated structures. This loss 

 appears to be caused by the kinetics of this region of the body, since if 

 anterior nodes are forced into the posterior they soon lose their power to 

 support full regeneration. We are therefore confronted with a two-way 

 interaction between the cytoplasmic particles and the macronucleus; 

 (i) the kinetics influence the nearby parts of the nucleus, causing the 

 posterior nodes to lose 'potency' ; (2) the macronucleus controls the mor- 

 phogenetic activity of the kinetics, so that the mouth and other anterior 

 organs cannot be formed in the absence of a fully potent nuclear node. 

 It would seem, therefore, that the kinetics cannot maintain their specific 

 character, or at least cannot continue to produce their specific eflect, with- 

 out the collaboration of nuclear factors ; their autonomy over against the 

 nucleus is by no means complete. 



4. Gene-initiated plasmagenes 



In contrast to the preceding categories there are a group of factors > 

 which are also often considered to be plasmagenes, and which are char- 

 acterised by the fact that they can arise anew within cells from which 

 they were originally absent. Their initiation seems in all cases to depend 

 on the functioning of corresponding genes in the nucleus and is impossible 

 if the effective gene is absent. Other conditions of an environmental kind 

 are usually necessary to bring the gene into play and cause it to produce 

 the cytoplasmic factor. 



In Paramecium, besides the kappa particles which have already been 

 described, there are cytoplasmic determinants of certain antigenic pro- 

 perties (Sonneborn and Beale 1949, Beale 195 1, 1952, 1954). We have 

 already (p. 3 59) referred to these in connection with the cytoplasmic control 

 of gene activity and they also illustrate other points which may be relevant. 

 Thus the cytoplasmic determinants (which are presumably particles, 

 though they have not yet been seen) are under close control by genes. A 

 given type of determinant cannot persist indefmitely in a cell from which 

 the corresponding gene is absent; and in this their behaviour is perhaps 

 similar to that of the Stentor kinetics. Further, the genes in the Paramecium 

 nucleus appear to be able to bring into existence the cytoplasmic deter- 

 minants which correspond to them, even if there were previously no 

 representatives of this particular type in the cell. Thus if Paramecia are 

 kept at 29-33° C. they will develop one of the D antigens (depending on 



