CELL DIFFERENTIATION 



523 



micro-en\ironment such that "outside "cells now find themselves" inside" 

 as in the case with buried neural tissue that once was external epithelium. 

 The three germ layers of the embryo, in fact, represent the outside and 

 inside of a hollow sphere respectively, together with those most anaero- 

 bically placed, the mesoderm that is literally sandwiched between its 

 epidermal and endodermal neighbours. A picture of sequential pro- 

 gramming thus arises, the order of the programming being vital in that 

 exposure to high-then-low levels of micro-environmental pCOo should 



morphogenetic 

 movement 



differentiation 



Fig. 6. Diagram to illustrate tht interrelatiuns between growth, morphogenetic 

 movement, and cellular differentiation (from Bonner [14]). 



cause a ditferent type of differentiated cell from one that has been exposed 

 to low-then-high. As in the education of a young man, exposure to 

 different environments can produce different final results. 



Why do cells move from one location to another within the developing 

 embryo ? One answer may be that they become positively or negatively 

 chemotactic to pCOo. Thus, Chlamydomonas cells are known to be chemo- 

 tactic to pCO., [15] and even the acrasin phenomenon in slime moulds may 

 be due to the mutual attraction of the washed amoebae owing to their 

 positive chemotaxis towards a high pCO., such as that generated by their 

 neighbours. Preliminary experiments in our laboratory have shown that 



