70 Cellular Differentiation 



The three bristle organ cells come from a single ancestor. At first, the 

 abdominal epidermis is simply a continuous sheet of many identical cells. 

 At a fixed stage in the development of the young adult from the immature 

 larva, single cells within the sheet grow to giant size. Division occurs and 

 one of the daughter cells is transformed into a sensory nerve cell. The 

 other daughter divides once more and one of these becomes the bristle- 

 forming cell, the other the socket-forming cell. See Fig. 35 for the lineage. 



It should be noted that a fixed number of giant cells appear in the 

 abdominal epidermis and always at specific points so as to yield a precise 

 pattern. There are mutant fruit flies, however, that display different num- 

 bers and patterns of bristles, and as one would expect in these organisms, 

 the number and positions of the giant cells differ from the norm. In addi- 

 tion, some mutants produce bristles of different shape and size or produce 

 none at all. These abnormalities, too, can be traced back to specific events 

 in the early development of the fly. For example, in one of the types that 

 produce no bristles, the giant cells arise at the right time and place and 



Fig. 35. Development of the bristle organ (adapted from C. Stern). 



2. Giant cell divides 



3. Divisions are complete, yielding 

 pre-nerve cell below and 

 parent of socket 

 ond bristle cells above 



4. Upper cell proceeds through mitosii 



5. Division of upper cell yields 

 the bristle and socket cells 



Socket 



Fiber leading to 

 /central nervous systen 



6. A semidiagrammatic drawing of the functional bristle apparatus 



