THE ACTIVATION OF GENES BY THE CYTOPLASM 355 



reaction (indication of -SH groups) is positive in all the nuclei of the 

 morula, but almost disappears from the ectodermal nuclei of the neurula, 

 although it still remains strong in the neural tissue and in the notochord 

 and mesoderm. Differences in the nuclei of the various regions of the 

 amphibian gastrula in the incorporation of amino-acids are described 

 on p. 204. Dounce (1954) has investigated the enzymes contained in nuclei 

 isolated from various adult tissues, and although the techniques of isola- 

 tion and handling of nuclei are not at present absolutely satisfactory, he 

 fmds strong evidence that the enzymatic properties differ considerably 

 between nuclei. On the whole the nuclei in a tissue are rich in just those 

 enzymes which are also found in the cytoplasm of the cells and it appears 

 that this cannot be due solely to the contamination of the preparations of 

 nuclei. Again Marshak (195 1) claims that the ribose nucleic acid (but not 

 the desoxyribose nucleic acid) of nuclei is very different in chemical 

 constitution in the various tissues. 



2. Effects on chromosomes and genes 



The formation in insect tissues of polytene nuclei, containing such large 

 chromosomes as we see in the sahvary glands, makes it possible to inquire 

 whether the fine-grain patterns of the chromosomes are similar in differ- 

 ent tissues. A well-formed polytene chromosome consists of a large 

 number of threads lying side by side, each thread consisting of an alterna- 

 tion of refractive and deeply staining segments with less deeply staining 

 stretches; the former contain much desoxyribose nucleic acid, the latter 

 much less. By cytogenetic methods, the position of a considerable 

 number of individual genes has been determined very closely; in fact it 

 may be possible to show that a gene must be located within a certain 

 particular deeply staining segment or band. If the activity of the genes 

 is different in different tissues, it might be that the appearance of the bands 

 would show some signs of this. The investigation of this point is com- 

 plicated by the fact that the classical type of 'salivary gland chromosome' 

 is the end-product of a long course of differentiation, and in the other 

 tissues of Drosophila the process does not usually go so far but stops at 

 an early stage in the sequence. Kosswig and Shengun (1947) were deceived 

 by this into the conclusion that the detailed structure of the chromosomes 

 is very different in salivary glands, Malpighian tubules, gut, etc. Slizynski 

 (1950) showed, however, that all the major landmarks of the chromo- 

 somes can be recognised in all tissues in which there is a reasonably good 

 development of polytene chromosomes, so that any differentiation of the 

 chromosome banding must be on a rather minute scale. 



Several workers, such as Beerman and Mechelke in Bauer's laboratory 



