Differentiation and Morphogenesis in Insects 247 



time, pupate in synchrony, and some days later they all emerge as 

 adult flies within a few minutes of one another. Therefore, as one 

 would hope and as Breuer and Pavan ( 1955 ) have confirmed, the 

 activities of their cells and tissues show a degree of synchronization 

 that is unmatched in any other metazoan. Each larva, in effect, is 

 equivalent to the next; so, in principle, one can design experiments 

 in which the same individual is sacrificed 150 times. 



As is the practice in most dipteran insects, the growth of larval 

 Rhyncliosciara takes place solely by cell enlargement, the chromo- 

 somes being duplicated again and again without any cell division. 

 Ordinarily, this would lead to a vast increase in chromosome number. 

 However, in Rhynchosciara as in most Diplera. the daughter chro- 

 matids do not separate following replication. Instead, they align them- 

 selves side by side to form compact, ribbon-like or cable-like 

 chromosomes up to 16.000 times larger than ordinary chromosomes. 



Within the giant chromosome each chromatid is a much-coiled 

 structure, the tightness of the coil dictating a periodic structure which 

 stains more intensely than do the adjacent loosely coiled regions ( Ris 

 and Grouse, 1945). And since the daughter chromatids pair "gene for 

 gene," the net result is that each giant chromosome can be recognized 

 under the compound microscope, not only by its overall size and 

 shape, but also by the pattern of its crossbanding. 



In Rhynchosciara there are four giant chromosomes in the nucleus 

 of each larval cell. The total number of crossbands is in the thousands; 

 in Drosophila mclanogaster (where the number has been counted) 

 there are 5,072, corresponding, it is alleged, to the number of gene 

 loci (Bridges, 1942). By taking advantage of the giant size of the 

 chromosomes, one may inquire as to whether a full and complete set 

 of genetic material is distributed to the various cell types during 

 embryonic development. 



In Rhynchosciara, as in Chironomus, the answer to this question 

 is clear-cut. In such diverse cells as those of salivary glands, midgut, 

 rectum, seminal vesicles, and Malpighian tubules, one can identify, 

 not only a full complement of giant chromosomes, but also the char- 

 acteristic fine structure in homologous parts of homologous chromo- 

 somes (Berger, 1940; Slizynsky, 1950; Beerman, 1952; Pavan and 

 Breuer, 1952 ) . 



On more detailed cytological examination, one finds that the giant 

 chromosomes of each type of cell are distinguished by the swollen 

 appearance or " 1 puffing ,, of certain specific bands. This is apparently 

 due to the uncoiling of the chromatids at these localized places — a 



