SECT. 5] 



IN ONTOGENESIS 



831 



Fig. 204. 



was a maximum e.m.f., but they were not able to offer any explana- 

 tion of this, nor have any more recent workers gone into the matter 

 anew. Their figures, which are presumably to be explained on a con- 

 centration-cell basis, are plotted in Fig. 204. Waller investigated the 

 "blaze currents" of the developing hen's egg. He had previously 

 defined a blaze current as an electrical response to some kind of 

 stimulus, whether electrical, chemical or photic. In its most charac- 

 teristic form it occurred in the same direction as the current by which 

 it had been excited, and this was important, for it could therefore 

 not merely be a polarisation counter-current. The blaze current, 

 according to Waller, is pre- 

 cisely analogous with the dis- 

 charge of an electrical organ, 

 excited by an electrical current 

 in the homonymous direction. 

 Having studied it in the eye- 

 ball and the crystalline lens, 

 he turned his attention to the 

 hen's egg, and, thinking that its 

 presence or absence might be an indication of whether the embryo 

 was living or not, he investigated a number of incubated eggs. As 

 he had expected, the blaze reaction made its appearance as develop- 

 ment progressed. Electrodes were brought into contact with the 

 shell-membrane, a small piece of the shell having been removed, 

 and whenever a blaze current appeared on stimulation, there a Hving 

 embryo was found when the egg was opened, and vice versa. 

 Waller confirmed the observation of Hermann & von Gendre 

 that there is a small current normally passing from egg-contents 

 to embryo, and observed also that by repeated excitations the 

 embryo can be killed or exhausted. Fig. 205 shows the decreasing 

 blaze current obtained from an egg which was stimulated several 

 times after 48 hours' incubation. Waller found that in the early 

 stages, before the blastoderm membrane had folded to form a 

 tubular embryo, the blaze currents were always positive, no 

 matter whether the excitation was positive or negative, but later 

 the blaze currents were always homodrome with the direction of 

 excitation, positive if the excitation was positive, negative if it was 

 negative. He extended these observations to frog's eggs, and reported 

 that for the most part definite blaze currents had been given by them. 



