THE TOPOGENESIS OF THE EMBRYO 105 



preferably at the side directed toward the original centre of 

 the grey crescent. From these observations, Dalcq and Pasteels 

 concluded that the occurrence of the potencies for gastrulation 

 in normal eggs is governed by (1) the yolk gradient, and (2) 

 the cortical field (see above p. 49). At any point, both the 

 "yolk factor" V, and the ''cortical factor'^ C, have a definite 

 value. Dalcq and Pasteels thought it probable that the product 

 of these factors, C X V, which also has a definite value at each 

 point of the egg surface, was a measure of the differences in 

 topogenetic potency. The first thought that comes to mind here 

 is that we are dealing with a chemical reaction between two 

 substances, which obeys the law of mass action. Where the 

 product C X V exceeds a certain value, the cells will acquire 

 a tendency to roll in. Where it remains below the liminal value, 

 there is a tendency for expansion in all directions. Where the 

 yolk concentration rises above a certain absolute value, the 

 cells remain more or less inert. Dalcq and Pasteels succeeded 

 in giving an elegant derivation, based on their hypothesis, of the 

 distribution of the topogenetic potencies in the normal blastula 

 (Fig. 38). 



We can summarise the above as follows. The chemodiffer- 

 entiation which occurs under the influence of the gradient- 

 systems governing the egg's polarity and symmetry, gives rise 

 to different topogenetic potencies in the various parts of the 

 egg. At a given moment, these lead to the occurrence of cell 

 migrations which result in gastrulation, and further topogenesis. 



