Parenchyma ce 



Fig. 36. The 



P. A. Weiss). 



Astrocyte 



lain cell types that arise from neural epithelial tissue (after 



each divides into two. One of the daughters as usual becomes a nerve cell 

 and the other divides as it should. But instead of yielding a bristle and a 

 socket cell, two socket cells appear! 



This work raises many questions. What genetic and biochemical 

 processes cause one cell among many suddenly to become a giant? Why 

 does it appear here and not there? Why, having produced three daughter 

 cells, does cell division stop? What causes three cells coming from the 

 same parentage to be transformed into totally different individuals? 



Cellular differentiation, as we see it in bristle formation, occurs in 

 every developmental system, whether it be slime mold, coelenterate, or 

 human embryo. Each stage of development is accompanied by the appear- 

 ance of new cell types which play a direct role in the formation of the 

 adult structure. Some of these cells, like the giant bristle cell in the fruit fly, 

 perform a specific act and then do one of three things: die, divide, or 

 simply become unimportant for further development, in which case they 

 can be ignored. Others, like the nerve cell in the bristle organ, persist and 

 play a role in the subsequent functioning of the adult. Figure 36 sum- 

 marizes a more comphcated series of cellular differentiation in the verte- 

 brate embryo. All the cell types shown arise in the neural tube in the 

 region of the brain. They are descended from ectodermal cells that in- 



71 



