CELLS IN DIVISION 



do not, however, support this view, for if the bands consisted purely of 

 coils in a spiral thread at microscopical orders of size they should be 

 obliterated by stretching the whole chromosome, in the way which is 

 described by Chambers^^^ for an early prophase chromosome of a sper- 

 matocyte of Dissosteira. However, neither chromomeres nor bands dis- 

 appear when salivary chromosomes are stretched, either if pulled out in 

 making smear preparations (Hinton^^^) or when the isolated chromo- 

 some is stretched by micromanipulation (Buck^*"). Most of the elonga- 

 tion occurs in the interband regions (Plate XVI). 



The technique of the isolation of salivary chromosomes was further 

 improved by D'Angelo.^^^ The main difficulty is that their physical 

 properties are altered by contact with torn cytoplasm; Buck found it 

 necessary first to treat the whole salivary glands with osmic vapour for 

 a day before dissection, but D'Angelo was able to avoid such pre- 

 treatment, and found that the chromosomes retained their natural 

 condition in a suitable mixture of sodium and potassium chlorides 

 with a phosphate buffer. Their elasticity is so great that they return to 

 their normal length even after a tenfold extension ; this requires that at 

 some level of sub-microscopical dimensions their structure must be made 

 up of folded or coiled units. D'Angelo was able to pull off fine fibrils 

 from the chromosomes in which the bands were represented by nodular 

 swellings at intervals. These fibrils were highly extensible and returned 

 to their original state on relaxation; D'Angelo does not mention 

 whether the nodular swellings persisted when the fibrils were stretched, 

 but it may perhaps be assumed that they did. 



Further evidence of the fine structure of salivary gland chromosomes 

 has been provided by electron micrography (Palay and Claude;^** 

 Pease and Baker ;i*3 Schultz et alii^**). All of these authors agree 

 that the bands of salivary chromosomes are made up of chromomeres 

 denser than the intervening chromonemata. Pease and Baker^*^ 

 describe cigar- or leaf-shaped bodies, within the bands, O'l^y. or less 

 in length, comparable with virus or bacteriophage particles and 

 which, they suggest, it is reasonable to regard as the genes them- 

 selves. However, although judgement on this point may be suspen- 

 ded, it may be agreed that most of the evidence on the structure 

 of salivary chromosomes suggests that the bands are made up of granules 

 arranged transversely and that the interband regions consist of fila- 

 ments, though it is still obscure what morphological status is to be 

 assigned to the latter. They are very clearly demonstrated in the replica 

 micrographs of Palay and Claude. 



Studies on chromosomes with the polarizing microscope have been 

 reviewed by Picken^^^ and by Frey-Wyssling.^*^ The birefringence of 

 fibres of nucleic acids is negative, while that of fibrous proteins is 

 positive; the appearance in polarized light of a structure composed of 



97 



