344 FIELDS AND GRADIENTS IN NORMAL ONTOGENY 



slightly shifted anteriorly, so as to alternate with those on the 

 cooled side: the precise meaning of this is not clear ^ (fig. 162). 



A curious effect upon cleavage has been noted in some of these 

 temperature-gradient experiments. It was not infrequently found 

 that in two sets of eggs from the same batch, one exposed to an 

 adjuvant and the other to an antagonistic gradient, the yolk-cells 

 were no more divided in the latter than in the former case, although, 

 of course, they had been exposed to a much higher temperature. 

 The cells of the animal hemisphere, on the other hand, were very 

 much smaller in the adjuvant series. In other words, the develop- 

 ment of the adjuvant series was more advanced, although its mean 

 temperature had been the same. This can only be explained by 

 postulating some effect of the rapid division of the heated animal 

 cells which stimulates division in other parts of the egg.^ 



§7. Differential susceptibility in the ontogeny of vertebrates 



Experimental modification of the primary gradient of the verte- 

 brate Qgg has also been achieved by the method of differential 

 acceleration. Certain treatments produce an acceleration of de- 

 velopment in all parts, but the acceleration is disproportionately 

 high in the more apical regions. For instance, by exposing the eggs 

 of the fish Macropodus to atropin sulphate for an hour and three 

 quarters during cleavage, the size of the head is increased relatively 

 and absolutely and it also has altered proportions, for the relative 

 width of the extreme anterior portion of the animal between the 

 eyes is much increased.^ Similar results have been obtained in 

 experiments on the frog,* notably with weak acids, and by means 

 of differential accHmatisation to very weak poisons (figs. 163, 164). 

 Equally interesting, and in some ways more instructive, results 

 have been obtained by the use of depressants on early stages, causing 

 differential inhibition. The depressant first used was magnesium 

 chloride,^ acting upon the fish Fundulus, and it was originally 

 thought that the effects were the specific result of that particular 

 substance ; but later work has shown that essentially similar effects 



^ Tazelaar, 1928. ^ Huxley, 1927; Castelnuovo, 1932. 



^ Gowanloch, in Child, 1924, pp. 85-6. 



^ Bellamy, 19 19, 1922. ^ Stockard, 1910. 



