Genes: Nafure and Mode of Action - 533 



AREA OF COHESION BETWEEN CELLS EQUALS 

 AREA OF ADHESION TO MEMBRANE 



COHESION DECREASED, 

 ADHESION INCREASED 



COHESION INCREASED, 

 ADHESION UNCHANGED 



COHESION INCREASED, 

 ADHESION DECREASED 



Fig. 27-6. How changes in cohesion between cells and adhesion to a supporting membrane may affect 

 the form of aggregated cells, in an embryo or in a tissue. It is assumed that no change has occurred in 

 the volume of the cells. (After Gustafson and Wolpert.) 



mining morphogenesis. But precisely how 

 these forces and factors are controlled by the 

 genes remains an open question. 



Special affinities of association between 

 cells, which seem to depend upon mutal co- 

 hesiveness, have also been demonstrated by 

 the work of Moscona and associates. The 

 technique of these experiments is to grow 

 mixed cultures of different cells in a common 

 fluid medium that is being stirred constantly. 

 The cells of a sponge, for example, tend to 

 recongregate, forming a complete organism, 

 after they have been subjected to experi- 

 mental treatments that cause them to be dis- 

 persed individually. The observations indi- 

 cate, however, that the reassociation is not 

 random. If the dispersed cells of a yellow 

 and red sponge are stirred in a common me- 

 dium, the yellows always aggregate with yel- 

 lows and the reds with reds. Or if liver, 

 retina, and kidney cells from a chick embryo 

 are circulating together in a common culture 

 medium, each kind tends to form aggregates 

 with its own kind. Moreover, in certain tis- 

 sues at least, definite points of attachment 

 may be formed between contiguous cells, as 



has been shown by the electronmicrograph 

 studies of Keith Porter of Harvard Uni- 

 versity. 



In some cases, specific substances may me- 

 diate an aggregation, causing mutual attrac- 

 tion between cells. One such substance, 

 acrasin, has recently been isolated by Shaffer. 

 This substance controls the aggregation of 

 cells in the slime mold, Dictiostelium (Fig. 

 2-23), which has been studied so intensively 

 by Bonner and others. Genie control of such 

 associative phenomena does not seem difficult 

 to understand, because genes are able to gov- 

 ern the production of specific enzymes, which, 

 in turn, determine the synthesis of specific 

 end products. 



Defermination. Sooner or later, each cell in 

 the embryo finally "takes its destiny into its 

 own hands" and becomes determined as to 

 its eventual differentiation. But the time at 

 which determination occurs differs widely 

 in different embryos, and in the different 

 cells of the same embryo. In a few embryos, 

 the cells are determined very early, even at 

 the two-cell stage — in which case each of the 

 separated blastomeres can give rise to only 



