38 THE CHROMOSOMES 



4,000 if one counts all the finer bands of which the 

 thicker ones appear to be made up.^^ Very frequently 

 (but not always) the bands are in pairs, that is to say 

 two adjacent ones are of exactly the same thickness 

 (Fig. 6). Each band is clearly a disk, that is to say 

 they extend through the thickness of the chromosome ; 

 moreover each disk or band is made up of a number 

 of granules which have more or less completely fused 

 to form a transverse plate ; in Chironomus there are 

 at least 256 of these granules in each band. The 

 granules in one band are connected with those in the 

 next by means of fine longitudinal threads which run 

 through the non- staining intemodes. The two strands 

 of our ' rope ' are thus themselves made up of a 

 number of threads which bear periodic enlargements 

 in the form of granules that tend (at any rate in 

 acetocarmine preparations) to fuse into transverse 

 bands.i9. ^2, 95 



This interpretation of the structure of salivary 

 gland chromosomes probably applies to all Diptera ; 

 various other theories as to their structure have been 

 put forward in connexion with genera (Drosophila 

 and Sciara) whose salivary gland chromosomes do 

 not appear to fix very satisfactorily in acetocarmine ; 

 but in larvae of midges (Chironomidae) there can be 

 no doubt that the above is the correct interpretation. 



The salivary gland chromosomes are thus to be 

 regarded as resting stage or early prophase chromatids 

 stretched out straight which are not wound into a 

 tight spiral as at an ordinary somatic metaphase and 

 which have split longitudinally again and again. All 

 the peculiarities of the salivary gland nuclei can be 

 explained on the basis of these two simple assumptions 

 if one takes into account the well-known somatic 

 pairing phenomenon found in all Diptera. The fact 

 that the salivary gland chromosomes are about fifty 

 times the length of the ordinary metaphase chromo- 

 somes will be understood if one takes a tightly coiled 

 spring and pulls it out straight. The great volume of 



