Hd Be 
Roux, W., 1903, Ueber die Ursachen der Bestimmung der Hauptrichtungen des 
Embryo im Froschei. ibid Bd. 23. 
ScHuLrze, O., 1887, Ueber Achsenbestimmung des Froschembryos Biol. Centralbl. 
Bd. 7. 
Scuutrze, O, 1887, Ueber die Entwickelung der Medullarplatte des Froscheies. 
Verhandl. physik..medic. Ges. za Würzburg. Bd. 23. 
Sumner, F. B., 1904, A Study of Early Fish Development. Arch. Entw. Mech. Bd. 17. 
Witson, E. B, 1892, The Cell lineage of Nereis. Journ. Morph. Vol. 6. 
Witson, H. V., 1900, Formation of the Blastopore in the Frog Egg. Anat. Anz. Bd. 18. 
5 , 1902, Closure of Blastopore in the normally placed Frog Egg. 
ibid. Bd 20. 
Physics. — ‘“J/lectrolytic phenomena of the molybdenite-detector.” 
3y M. J. Huizinca. (Communicated by Prof. Haga) 
(Communicated in the meeting of September 27, 1916) 
In an inquiry into the cause of the unipolar conduction of erystal- 
contacts, which are used as detectors in the technics of wireless 
telegraphy, it was tried to demonstrate the existence of a secondary 
E. M. F. when the primary current had ceased. 
To this purpose a current of some milliamperes was for some 
time sent through a erystal-contact, the electrodes of which were 
connected to a galvanometer after stopping the current. This experi- 
ment was repeated with a reversed primary current. Whereas the 
deflection of the galvanometer in all other combinations inquired 
into was not worth mentioning, it was very large in a molybdenite- 
brass detector; even when the primary current had passed during 
part of a second only. This deflection changed, both in direction 
and magnitude, when the primary current was reversed. An electro- 
meter taking the place of the galvanometer indicated 0,7 volt; as 
the five-cellular quadrant-electrometer, which was used, has a large 
capacity, the potential difference between the electrodes is undoubted- 
ly larger; such an E. M. F. can hardly be explained by thermo- 
electric forces. 
When, after many experiments with the same contact, a small 
dark-coloured spot had begun to show round about the brass point, the 
place of contact was observed during the passage of the primary 
current in a microscope magnifying thirty times. 
It proved to be very fit to let a piece of the mineral float on 
mercury and to put upon it a platinum point with slight pression. 
If, now, a current of some milliamperes is sent through the contact 
from J/oS, to Pf, then, after some moments, in some cases after 
