SECT. 5] IN ONTOGENESIS 827 



the same curve. I have not figured Gayda's points for the period 

 after the 30th day from fertilisation, for, with the initiation of 

 metamorphosis, the embryological sphere is transgressed, but it may 

 be said that the electrical resistance is gradually lowered, until at 

 the time of completion of the posterior legs it has attained a practically 

 constant value. This equates extremely well with the behaviour of 

 the total ash as shown in Fig, 230. The amperage and voltage of the 

 current between head and tail continue to increase more or less 

 regularly through metamorphosis. The difference of potential be- 

 tween the tail end and the head end suffers an extremely rapid 

 change over at about the 95th day from fertilisation, i.e. just before 

 the completion of metamorphosis ; before that time the tail end was 

 always about 10. lO"^ volts higher potential than the head end, but 

 after it the reverse relation held. Gayda concluded that the embryo 

 could be pictured as a series of solutions of diverse concentration 

 and constitution, separated by semipermeable membranes^, and that 

 changes in morphology in such a system would have the effect of 

 setting up small currents such as he had measured. 



Mendeleef has concerned herself much with the comparative elec- 

 trical resistance of tissues. Using PhiHppson's method for determining 

 the electrical resistance of tissues, which consists in measuring the 

 resistance a known amount of tissue opposes to the passage of an 

 alternating current of frequency varying from 1000 to 3,000,000 

 periods per second, produced by a thermionic valve, she investi- 

 gated the magnitude of PhiHppson's constants for embryonic and 

 adult tissue. These constants are (a) R, the specific resistance of 

 the cell-contents, i.e. the reciprocal of the electrolyte-content of the 

 cytoplasm, (b) r, the resistance per cubic centimetre of tissue, i.e. 

 inclusive of intercellular spaces, membranes, etc., and (c) p, the 

 resistance in ohms per cubic centimetre of tissue corresponding to 

 polarisation at a frequency of zero. For the liver of the normal 

 guinea-pig, these were as follows: 



Ohms 



R 195 



r ... ... 1790 



P 4-56 . lo^ 



During pregnancy, R was lowered by not more than 5 per cent., 

 r by about 15 per cent. a,nd p by 50 per cent. Mendeleef concluded 



1 Responsibility for the resistance must probably be placed more on the membranes 

 than on the ash-content. 



