594 ON INCREASE IN COMPLEXITY [pt. iii 



toad tadpoles to be galvanometrically negative to the heads, and 

 this Hyman & Bellamy confirmed for the frog. "The idea is ad- 

 vanced", they said, "that differences of potential in organisms, 

 particularly the permanent differences which exist along the main 

 axes of animals, are due to differences in metabolic rate at different 

 regions, the region of highest metabolic rate being the most negative 

 in the external circuit, most positive in the internal circuit." In the 

 tadpole, therefore, the highest physiological level appeared to lie 

 towards the tail. 



Fig. 98. 



^. 



Fig- 99- 



Hyman has published a series of papers on the susceptibility 

 gradients of vertebrate embryos. The first of these she devoted to 

 the teleost embryos {Fundulus heteroclitus (minnow), Ctenolabrus ad- 

 spersus (cunner) and Gadus morrhua (cod)). She showed that, in 

 addition to the primary gradient of the embryo which has its high 

 level pole at the anterior or cephalic end, there were also other 

 "secondary" gradients arising from regions of high susceptibility 

 other than the anterior end. Certain organs, also, may have their 

 own axiate pattern, notably the heart. 



The cod and cunner embryos were studied with cyanide in the 

 usual manner, but the impermeable egg-membranes of the minnow 

 made this impossible, so that ammonium hydroxide had to be used 

 instead. In the early blastoderm stages of the cunner and the minnow, 

 the central cells were observed to be the most susceptible, and from 

 them disintegration proceeded to the periphery of the blastoderm. 

 But in the case of the cod exactly the reverse relationship held true ; 



