Cellurar 

 Differentiation 



rofessor Curt Stern, one of the fathers of mod- 

 ern genetics, in his article ^ entitled "Two 

 or Three Bristles," has a superb discus- 

 sion of cellular differentiation that is de- 

 signed to be read by scientists who are 

 not biologists; it can easily be understood 

 and will be greatly appreciated by seri- 

 ous beginning students of biology. 



To illustrate the problems inherent 

 in cellular differentiation, Dr. Stern de- 

 scribes the formation of bristles upon the 

 abdomen of the fruit fly: 



In some regions there arise short or 

 long outgrowths— the bristles— strong and 

 wide at the base and gently tapering to 

 a fine point. Narrow grooves, as in fluted 

 columns with a slightly baroque twist, 

 extend along their lengths. A short stalk 

 fits each bristle into a round socket within 

 the body armor so that the bristle can be 

 moved within this articulation . . . The 

 bristles are tiny sense organs, perhaps 

 sensitive to the fluctuations of air pressure 

 when the fly is in flight. 



Dr. Stern goes on to describe the 

 cellular structure of the bristle organ. It 

 consists of three cells: the bristle cell it- 

 self, which secretes the tapered out- 

 growth; the socket cell that secretes a 

 socket-like ring of hard chitin into which 

 the base of the bristle fits; and, below 

 these two, a sensory nerve cell that is 

 linked to the bristle by a short nerve 

 fiber and whose other long nerve fiber 

 connects up with the central nervous sys- 

 tem, thereby communicating to it stimuli 

 felt by the bristle. 



1 Published in American Scientist, April 1, 

 1954. 

 69 



