-SECT. 5] IN ONTOGENESIS 825 



They felt, therefore, that they had explained why the yolk always 

 floats with the germinal disc upwards, and they suggested that the 

 oily substances were more concentrated there. This explanation is 

 difficult to accept, in view of what we know about the relations 

 between the white yolk and the yellow yolk. Baudrimont & Martin de 

 St Ange drew a tentative parallel between the oriented floating 

 of the hen's egg-yolk and the animal and vegetal poles of the 

 frog's egg. Among the few pieces of work on this subject is that of 

 Mussehl & Halbersleben and Dinslage & Windhausen who found 

 that the specific gravity of different individual batches of eggs bore 

 no relation to the percentage hatch. Groebbels has shown in the 

 case of a number of wild birds' eggs that the specific gravity decreases 

 during the course of development: in some instances this process 

 goes exactly inversely to the weight of the embryo. 



5'8. Potential Differences, Electrical Resistance, Blaze Cur- 

 rents, and Cataphoresis 



Some further remarks must now be made on the subject of the 

 electrical properties of embryos. A certain number of investigators 

 have found a constant potential difference between the head and the 

 tail ends of various embryos, and their work was reviewed by Hyman 

 & Bellamy in 1922. Hyde's work on the potential of sea-urchin 

 embryos may be mentioned — her experiments on frog embryos were 

 confirmed by Viale. 



More recently work on these lines has been carried on by Gayda. 

 Leading off through electrodes placed at the poles of the embryos, 

 he found the potential differences to be as follows : 



More detailed figures are shown plotted in Fig. 202. It is very 

 striking to notice the way in which the electrical resistance rose con- 

 tinuously. More examples of this phenomenon will shortly be given, 

 and it seems to be, indeed, a general rule that the electrical resistance 



