TRANSLOCATION 133 



enzymes whieh will in turn carry out a process of catalysis or 

 synthesis in the animal cell. In this connection it is not unjustifi- 

 able to point out that in plant cells there is ev^ery reason to believe 

 that the chief cell organ of synthesis — the plastid system — is 

 phylogenetically related to the chondriome of plants. 



THE NUCLEUS AND NUCLEOLUS 



We pointed out in the introduction that the main phenomena 

 connected with the nuclear cycle were elucidated by the end of 

 the w^ar. Certain problems such as the demonstration cyto- 

 logically of crossing over, and the possibility of parasynapsis and 

 telosynapsis are always present, but they have faded rather 

 from the cytologist's view during the present decade. It is true, 

 however, that the work of Hogben (55-57) did much to demon- 

 strate parasynapsis in animal cells whatever the conditions found 

 in plants may be. 



The most iixiportant feature of the cytology of the nucleus in 

 recent years is the demonstration by the school of Muller in 

 America of " translocation " and its related phenomena (86-88, 

 103). When chromosomes in germ cells are exposed to X-rays in 

 appropriate doses they are found to exhibit a tendency to 

 fragment. This phenomenon has been known for a good time, 

 but it now appears that these fragments, although they may 

 remain separate, often become attached to one of the other non- 

 homologous chromosomes. This rearrangement of the chromo- 

 somal material is stated by the geneticists to be associated with 

 differences in the genetic behaviour. It is found that some of 

 the mutations normally associated together (linked) in breeding 

 experiments are now associated with different groups of mutations. 

 A complete correspondence between the genetical facts and the 

 cytological observations is difficult to establish in Drosophila on 

 account of the similarity in appearance of the second and third 

 chromosomes. But it appears that similar alterations in the 

 linkage groups occur between the autosomes and the sex-chromo- 

 somes. Thus, a translocation from III to Y (Fig. 61, C) has 

 enabled a distinction to be made between the second and third 

 chromosomes. This establishment of the third chromosome as 



