206 NUCLEAR TRANSPLANTATION 



polyteny in different cells might also be taken as evidence of 

 quantitative multiplication of whole sets of the basic genetic 

 units in the nuclei containing them. 



The constant relation of definite polytene and polyploid num- 

 bers to certain insect tissues strongly suggests that they are re- 

 lated to differentiation or special functions in these cells. White 

 (1945, pp. 33-34) expresses the opinion of a number of cytolo- 

 gists in attributing developmental significance to these nuclear 

 tissue-specific relationships when he suggests that "The whole 

 process of histological differentiation in insects seems to be inti- 

 mately bound up with this phenomenon of endopolyploidy, each 

 organ and each tissue having its own characteristic degree of 

 ploidy, some being entirely composed of one type of cell while 

 others are mosaics of cells with different multiples of the funda- 

 mental diploid number. To what extent endopolyploidy occurs 

 outside the Insecta is not known at present, but there are indica- 

 tions that it is fairly widespread in many groups of animals." For 

 a critical evaluation of this possibility, one is also referred to 

 Huskins (1947). 



In addition to the presence of quantitative differences in the 

 number of complete sets of chromosomes in different somatic cells, 

 there is also evidence for tissue-specific variation in the structure 

 of individual chromosomes. Kosswig and Shengiin (1947) and 

 Beermann (1952) working with Chirouomiis larvae have made 

 detailed cytological studies of similar regions on homologous 

 chromosomes obtained from salivary glands, Malpighian tubules, 

 mid-gut, and rectum. They found major tissue-specific differences 

 in polyteny, in overall size of comparable regions and, most note- 

 worthy, differences in the thickness and number of homologous 

 discs at specific loci on the chromosomes. Moreover, these varia- 

 tions showed modification with developmental age with some dis- 

 tinctive structural features appearing or disappearing at various 

 times during differentiation. These observations are contrary to 

 the less extensive, but somewhat comparable, study of Berger 

 (1940) on salivary and mid-gut chromosomes of Sciara in which 

 similar regions of homologous chromosomes from the two tissues 

 were described as having similar structure. The work of Kosswig 



