Abstract
It is well known that the amphibian organizer center (dorsal blastoporal lip) induces a neural plate in any presumptive ectoderm with which it may come in contact. It has been claimed that neural plates may also be induced in presumptive ectoderm by application of a thermal gradient (Gilchrist, 1 Castelnuovo 2 ) as would be expected on the basis of Child's 3 gradient theory. Our experiments on the tree frog (Hyla regilla) have not thus far substantiated the latter work.
The apparatus used, we believe, is easily the most accurate yet applied to this study of the effects of thermal gradients on amphibian eggs. The thermodes used were made of 4 coils of platinum wire 1/500 of an inch in diameter, insulated with a thin coat of lacquer, and heated with current from a storage battery. The temperature could be regulated by a resistance box. The coils of platinum wire were carefully placed against the upper surface of the egg by means of a micro-manipulator set. It was possible to observe the eggs constantly during the period of heating. The eggs were also vitally stained where the thermode was placed, in order to be certain that no movement took place.
The temperature applied to the egg surface was ascertained directly by the use of a micro-thermocouple and galvanometer.
The thermal death-point of the cells was found to lie at about 36-38° C. Our experiments to date have been concerned with the application of temperatures close to the thermal death-point (27-36° C). On the basis of the physiological gradient theory of Child, these high temperatures ought to be most effective in inducing secondary neural structures.
Examination of the external surfaces of the heated eggs has thus far revealed no case of clear neural induction.
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