356 PRINCIPLES OF EMBRYOLOGY 



in Germany and Pavan in Brazil, have recently discovered species in 

 v^hich the polytene chromosomes are well enough developed in a variety 

 of tissues for their structure to be studied in detail. They have found clear 

 evidence that individual bands, which may correspond to single genes, 

 may be differently developed according to the tissue in which they lie; 

 moreover some bands can be shown to pass through characteristic cycles 

 of change, which appear to indicate the occurrence of important metabolic 

 events at particular times in development. This is critically important 

 evidence for the supposition, which has always seemed reasonable on 

 general grounds, that the degree of activity of a gene depends on the 

 cytoplasm surrounding it, and varies not only from tissue to tissue, but 

 with the epigenetic situation within any one tissue. 



Beerman (1952) has recently made a detailed study of the polytene 

 chromosomes of several tissues of the species Chironomus tentans in which 

 they are very well developed. He fmds that there are characteristic differ- 

 ences in the general appearance of the chromosomes in the tissues studied; 

 in the salivary gland they are rather compact cylinders, in the Malpighian 

 tubules and rectum they are peculiarly kinked, and in the midgut they 

 have the form of spirally wound flat strips. In all tissues, however, the 

 same sequence of bands can be recognised, so that in all cases the comple- 

 ment of genes appears to be complete, as far as the cytological evidence 

 goes. It is very important to observe that individual bands show character- 

 istic appearances in the different tissues. This can be seen in Fig. 16.4, 



Figure 16.4 



A certain region in the third chromosome o£ Chironomus tentans, identifiable 

 by a small inversion which causes a failure of pairing: a is from a cell in the 

 mid-gut; h, salivary gland; c, Malpighian tubule; d, rectum. Note that the 

 bands (or group of bands) 1-6 are identifiable in every case, but may be 

 differently developed in different tissues. (After Beerman 1952.) 



