THE ACTIVATION OF GENES BY THE CYTOPLASM 



359 



A very remarkable example of cytoplasmic activation of genes, and 

 one very hopeful for future investigation, occurs in the protozoan Para- 

 mecium. In this animal, there may be found one or other of a series of anti- 

 gens, v^hich can stimulate the formation of corresponding specific anti- 

 bodies when injected into rabbits. Paramecium has one great advantage as 

 an experimental animal in that it is possible to arrange for a nucleus of one 

 genetic constitution to get into cytoplasm which has been formed mider 

 the control of a nucleus of a different type or under the influence of differ- 



Neural 

 tube 



Notochord 



Figure i6.6 



1. An egg of Triturus palmatus when fertilised by sperm of T. cristatus 

 develops normally. 



2. If the palmatus egg nucleus is removed before fertilisation, we obtain a 

 'hybrid merogon', with a haploid cristatus nucleus in palmatus cytoplasm; 

 this dies at the early neurula stage. 



3. The hybrid merogon combination o£ palmatus cytoplasm with alpestris 

 nucleus dies rather later, while {4) the reciprocal combination of palmatus 

 nucleus in alpestris cytoplasm dies earlier. On the right is a diagrammatic 

 longitudinal section through a hybrid merogon of cristatus nucleus in 

 taeniatus cytoplasm (which behaves like the cristatus in palmatus combination 

 shown in Figure 2). The dots show the region in which the mesoderm 



cells become necrotic. (From Baltzer 1940.) 



ent environments. This may occur during the process of conjugation, in 

 which two Paramecia, possibly of different genetic constitution, come 

 together and each inseminate each other. Very little if any cytoplasm is 

 normally exchanged during this process (though it is possible to arrange 

 for this to happen if the experiment requires it), which therefore gives 

 rise to two individuals, each of which contains a similar hybrid nucleus, 

 but each with its original characteristic cytoplasm. 



Experiments of this kind (Sonnebom and Beale, 1949, Beale 1952, 

 1954) have shown that the immediate control of antigen formation is due 



2a 



