494 APPENDIX 



2. Lateral temperature-gradients and gradient-fields in 

 Amphibia. 



Gilchrist {Jouni. Exp. Zool. Lxvi, 1933, p. 15) has recently re- 

 attacked the question (raised on p. 342) as to whether the gradient- 

 system in the amphibian embryo can be directly altered and 

 deformed by means of lateral temperature-gradients so as to give 

 rise to asymmetrically developed neural folds, independently of the 

 effects of alteration of growth of the invaginated organiser. A new 

 method introduced consists in applying a lateral temperature- 

 gradient for a certain (not too great) length of time, and then to 

 reverse the sign of the gradient for an equal length of time. Eggs of 

 the Urodele Tritiinis thus treated, and heated first on the right 

 during the late blastula and then on the left during the early 

 gastrula stages, show abnormally large neural folds on the right 

 side. 



Gilchrist draws the conclusion that the processes of neural plate 

 determination (in what we should call the primary gradient-system) 

 take place during the late blastula, for which reason the temperature- 

 gradient to which they are then exposed is able to bring about a 

 larger development on the heated side. This view receives support 

 from the results of other experiments in which the temperature- 

 gradient is applied earlier : heated on the right in the early blastula, 

 and on the left in the late blastula. In such cases the neural folds 

 are larger on the left. 



We are, however, not informed as to whether the invaginated 

 gut-roof is symmetrical or not, and it would still be possible to hold 

 that the late blastula stage is the critical time for neural plate 

 determination, not because of any effect on the gradient-system, 

 but because the processes of invagination of the organiser (gut- 

 roof) are also susceptible to modification by temperature at this 

 period. In this case, the effects of the temperature-gradient on the 

 neural folds would be indirect, and exerted via the organising 

 action of the gut-roof. Against this, however, it must be mentioned 

 that Gilchrist presents evidence of the early blastula stage as being 

 that at which the embryo is most susceptible to temperature- 

 gradients for the production of abnormalities in the subsequent 

 processes of gastrulation. 



