GRADIENT-FIELDS 43 



cooled at the other). On the other hand, a gradient system may 

 extend over a flat or curved surface, the quantity concerned 

 decreasing, for example, in all directions from a central maxi- 

 mum (for instance: atmospheric pressure gradients at the 

 surface of the earth). This is called a surface gradient. Finally, 

 a gradient may extend in all directions in space, e.g. the con- 

 centration gradient of a salt crystal dissolving in water. This 

 is called a spatial gradient system. 



Now in many eggs gradients have been found which show 

 some connection with polarity or symmetry. We have already 

 mentioned that there is an increase in yolk concentration along 

 the main axis of many eggs (p. 39). This may be called a yolk 

 gradient. Further, Child and his collaborators have for many 

 years stressed the importance of local differences in the in- 

 tensity of metabolism, as expressed in differential sensitivity 

 to noxious influences in different regions. In animal eggs, as 

 a rule the sensitivity is greatest at the animal pole, and from 

 there it decreases gradually in the direction of the vegetative 

 pole. This points to a variation in metabolism of the egg along 

 its main axis. Similar results have been obtained in determin- 

 ations of the redox potential (rH) of the egg cytoplasm with 

 various dyes, such as Janus green and methylene blue (Ries 

 and Gersch, 1936). 



In this context experiments by Spek (1930-1934) deserve 

 mention. He measured the degree of acidity (pH) of proto- 

 plasm by means of indicator dyes (substances whose colour 

 depends on the pH of their environment), e.g. neutral red, nile 

 blue hydrochloride and brilliant cresyl violet. Initially, most 

 eggs showed a more or less uniform coloration, but shortly 

 after the beginning of development a ''bipolar differentiation" 

 took place, i.e. the colours of animal and vegetative regions 

 began to differ. The colour of the animal cytoplasm indicated 

 an alkaline reaction, that of the vegetative part of the egg an 

 acid reaction. Spek explained this as follows. Originally colloid 

 particles with positive and negative electrical charges were 

 mixed throughout the egg. Later, the positive particles would 

 move to one pole, and the negative particles would migrate 

 to the other pole. The mixture would thereby be separated, and 



