INDIVIDUATION — FORMATION OF PATTERN AND SHAPE 445 



able to show very clearly that the adhesiveness of a cell for other cells of 

 the same or different kinds may change considerably during the course of 

 differentiation. Thus in some tissues the cells are very closely packed at 

 certain stages, when their adhesiveness is high, but tend to become dis- 

 sociated from each other at other times, as for instance when the neural 

 crest cells break away and start to migrate separately between the ecto- 

 derm and mesoderm. Again, combinations of different tissues sometimes 

 show considerable mutual affinity, when they round up into a single mass, 

 while at other stages the two tissues may tend to separate from one 

 another (Holtfreter 1939). Such processes are undoubtedly very important 

 in influencing the shapes of neighbouring masses of tissues in a developing 

 embryo. Holtfreter has given a number of diagrams illustrating the effects 

 of these mutual interactions on the forms assumed by tissues some way 

 along in their differentiation (Fig. 20.15). 



At the stage of gastrulation, differentiation has not proceeded very far 

 and one would not expect to fmd clear-cut differences in affinity, but 



FiGxiRE 20.15 



Figure a, isolated ectoderm develops into an irregular epidermal tissue; b, 

 ectoderm combined with some mesenchyme forms an epithelium ciliated 

 on the outer surface ; c, ectoderm embedded in mesenchyme, inside another 

 epidermal skin, forms a vesicle or tubule ciliated on the inner surface. (From 

 Holtfreter 1939.) 



instead more gradual transitions between one region and another. Holt- 

 freter gives some evidence that such graded differences exist. He is in- 

 clined to interpret them as differences in surface tension. He points out 

 that if one has two drops of different materials, A and B, in contact and 

 both immersed in a medium C, and if the tension in the AC surface is 

 greater than the sum of the tensions in the BC and BA surfaces, then the 

 AC surface will contract, and the drop of ^ will in effect be engulfed by 



