PLASMAGENES 395 



with a single cilium may, during regeneration, take part in the formation 

 of a gullet or contractile vacuole or any other organ. The kinetics are in 

 fact organised in relation to one another in a 'gradient-field', somewhat 

 reminiscent of that seen in Platyhelminthes, for instance. Thus there is a 

 leading kinety (associated usually with the mouth), to which the rest are 

 subordinated as the hind parts of a flatworm are subordinated to the head. 



The fact that one and the same kinety may, under the conditions of 

 regeneration or reorganisation, produce different structures, must mean 

 that the effect of the kinety depends on the properties of the cytoplasm 

 with which it is in contact. The situation is comparable with that wliich 

 we have discussed in relation to the nuclear genes ; the kinetosome may 

 continue to dupHcate itself identically but at the same time interact with 

 the local cytoplasm, producing different active products according to the 

 raw materials or other substances available. The organisation of the kinetics 

 into a gradient field is presumably brought about by the leading kinetics 

 affecting the cytoplasm in some way which diffuses outwards and in- 

 fluences the activities of the subordinated kinetics. 



Besides these mutual interactions between the various cytoplasmic 

 particles, there are very interesting relations between the particles and 

 the nuclei. Weisz (195 1) has studied the matter in the particularly favour- 

 able case of Stentor. This is a large sessile ciliate, which possesses not only a 

 micronucleus but also a large macronucleus which consists of a string of 

 swollen nodes connected by a much thinner strand. The micronucleus 

 seems to be concerned solely with sexual reproduction, and a Stentor 

 from wliich it is missing can live for many generations of vegetative 

 fission, and regenerate quite adequately (in some other cihates the micro- 

 nucleus is necessary for regeneration). These processes are, in fact, under 

 the control of the macronucleus. The genetic constitution of this is not 

 known with any certainty, but it seems likely that it is to be considered 

 as a highly polyploid nucleus in which the original set of chromosomes 

 has been multiphed many times; probably each node contains at least one 

 diploid set of chromosomes and possibly more (Fig. 18.5). 



During fission of a Stentor into two daughter individuals, the macro- 

 nucleus also undergoes reorganisation, and in the period shortly after this 

 it is found that all the nodes are equivalent, in the sense that any one of 

 them is sufficient to make possible the regeneration of a whole individual. 

 Later in the life-cycle, as the time approaches for the next fission, this is 

 no longer the case. If at this stage a fragment is cut off a Stentor in such 

 a way as to include only a posterior node, regeneration is not complete 

 and a mouth is not formed. This is so even if the cytoplasm comes from 

 an anterior region and is known to be capable of carrying out a complete 



