218 CELLS, TISSUES, AND ORGANISMS 



mental steps toward the study of cell interactions and organization; and 

 that the means for their exploration are available in the methodology of 

 tissue synthesis from dissociated cells. 



In summary: In vitro methods aflFord advantageous experimental 

 approaches to various aspects of cellular interactions in relation to his- 

 togenetic processes. Recently available procedures make it feasible to 

 compound dissociated cells, in various combinations and under rigor- 

 ously controllable conditions, into developmentally effective multi- 

 cellular systems. The events of the process and the products of cellular 

 aggregation can be evaluated in precise terms. Cell aggregates vary 

 characteristically and consistently with different types of cells and 

 stages of development. Histogenetic cohesion of cells has been found 

 to depend on thermo-sensitive processes. It is suggested that morpho- 

 genetic cell contacts are mediated by specific products elaborated by 

 cells and accumulating between them. It is further proposed that such 

 extracellular materials may be involved in the processes of sorting out, 

 preferential convergence, and type-specific grouping of cells in aggre- 

 gates, by providing a cell-linking continuum through which distance 

 interactions may take place. 



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