VVe have seen the complexities that attend the 

 construction of a multicellular organism, 

 whether it be sHme mold fruiting body 

 or vertebrate embryo. Many new cell 

 types arise and must maintain fairly rigid 

 numerical proportions. Cells must mi- 

 grate to specific areas of the aggregate 

 and must be integrated into tissues and 

 organs. All these processes are underlain 

 by a bewildering array of biochemical 

 activities that must occur in strict chron- 

 ological order. 



How does the system maintain a 

 balance of kind, number, place, and 

 time? Obviously, each subunit of the or- 

 ganism cannot be permitted to develop 

 autonomously, disregarding other sub- 

 units. Each, therefore, must receive in- 

 formation from other parts about what 

 these have been doing in the immediate 

 past and about what it may do and must 

 not do in the immediate future. We will 

 now concern ourselves with the exchange 

 during Growth between cells of chemical messengers 



that trigger, assist, control, and inhibit 

 developmental events. 



Cell 

 Interactions 



and 

 Morphogenesis 



Many Cells Do What Few Cannot Do 



The simplest example of this kind 

 of interaction is seen in cultures of micro- 

 organisms or dispersed animal or plant 

 cells growing in a liquid nutrient me- 

 dium, a topic we will take up in detail 

 in the next chapter and briefly describe 

 here. Such cells, when inoculated into 

 fresh medium, experience a lag of vari- 



