198 CELLS, TISSUES, AND ORGANISMS 



However, it is becoming increasingly clear that a full account 

 of developmental processes cannot be expected without considerably 

 more information about the cellular level of organization— about the 

 functional properties of the living units that make up living fabrics— 

 their behavior, their molecular requirements, and their products under 

 various conditions, i.e., their spectrum of reactions and interaction. In 

 this connection there is also a growing recognition that investigation of 

 developmental phenomena solely by dissection, isolation, and analysis- 

 morphological or biochemical— can result, at best, only in partial 

 notions; that, by analogy with the methodology of chemical sciences, 

 analysis of developmental processes must go hand in hand with synthe- 

 sis, with attempts to construct orders of higher complexity from simpler 

 units. As Conklin (1951) optimistically pointed out: "If we are ever to 

 comprehend the nature of life, we must employ synthesis as well as 

 analysis." Thus, in searching for the basic laws and factors involved in 

 bonding and organization of cells into tissues one might, perhaps, strike 

 fastest at the core of these problems by attempting to combine living 

 cells into tissues— to synthesize multicellular systems from individual 

 units. In the past few years we have attempted to translate these inten- 

 tions into concrete experimental propositions, to learn the fundamen- 

 tals of cell interactions, and thus to pursue a cellular approach to the 

 study of tissue development. Some of the results of this exploratory 

 spadework provide the background for this discussion. 



Self-aggregation of cells 



We might begin by briefly recalling the evidence that first sug- 

 gested to us that attempts in this general direction were feasible. The 

 main impetus was provided by the phenomenon of self-aggregation 

 of dissociated embryonic cells. Some years ago it was found that vari- 

 ous tissues and organ-rudiments of mouse and chick embryos could be 

 readily dissociated into individual, viable cells in suspension by treat- 

 ment with cation-depleting agents and with enzymes ( Moscona, 1952 ) . 

 Such dispersed cells, when maintained in an appropriate liquid culture 

 medium, did not remain separate. Having settled on the floor of the 

 culture container, they moved about, colliding and joining into numer- 

 ous small clusters or aggregates. These primary aggregates increased in 

 size by accretion of free cells and by fusion, some of them reaching, in 

 less than 24 hours, macroscopic dimensions. Their rate of formation 

 varied with different types of cells, the cell concentration, the culture- 

 medium composition, and various environmental and cellular condi- 

 tions, which were not always easy to define, standardize, or control. 

 There was thus considerable variability in the details of the process. 

 However, on the whole the major result was always consistent: having 



