446 



PRINCIPLES OF EMBRYOLOGY 



the drop of B. This, Holtfreter suggests, may provide a model of the 

 incorporation of groups of cells placed in contact with endoderm, and 

 indeed of one of the factors involved in the normal invagination (Fig. 

 20.16). 



The general principle of the suggestion seems rather plausible, but some 

 caution is advisable in using the term 'surface tension' in connection v^ith 

 amphibian cells. Strictly speaking, a surface tension develops only in a 

 liquid interface, and there seems httle doubt that the external membranes 



Figure 20.16 



Surface tensions in two drops of different fluids A and B immersed jji a 



third C. 



of cells cannot be regarded as truly liquid, but must be supposed to possess 

 a certain degree of rigidity or solidity. This does not make it impossible, 

 however, for such surfaces to exhibit properties analogous to those of the 

 tensions which would develop in truly liquid interfaces. Returning to 

 the two drops A and B in contact with one another, the engulfment of ^ 

 would also occur if there was a strong tendency for the area of contact 

 between A and B to increase, and such a tendency would arise if the A 

 and B membranes in some way attracted one another. Since the mem- 

 branes are largely protein, they may be expected to exhibit an orderly 

 disposition of chemically reactive groups, which will tend to become 

 attached to the appropriate groups on some neighbouring surface if the 

 two patterns fit, but not otherwise. 



Weiss (1947, 1949, 1950b) has emphasised the important part that may 

 be played in development by the mobilisation at the surface of the cell 

 of compounds that have a specific chemical reactivity which causes 



