90 
PHYSICS: D. L. WEBSTER 
and give rise to cells which are more or less removed from their normal 
places. Thus polar bodies and ectoderm cells may be caused to form 
at the vegetative pole or at any other point on the surface of the egg 
instead of at the animal pole; nevertheless the protoplasmic portions of 
the yolk-containing cells return to the animal pole when centrifuging 
ceases unless otherwise prevented. In some instances in which centri- 
fuging took place in the two cell stage the normal positions of proto- 
plasm and yolk are regained in one of the cells but not in the other. 
All such cases indicate that it is difficult but not absolutely impossible 
to change the polarity of eggs and cleavage cells, and that the persistence 
of polarity in centrifuged eggs and the restoration of dislocated parts 
to their normal positions is connected with a somewhat resistent frame- 
work of protoplasmic strands which preserve the relative positions of 
nucleus and centrosphere in the cell axis. 
Child, C. M., Studies on the Dynamics of Morphogenesis, etc. I-V, VII-VIII. /. Exp. 
Zool, 10, 11, 13, 14, 16, 17 (1911-1914). 
Conklin, E. G., Effects of Centrifugal Force on the Organisation and Development of 
the Eggs of Fresh Water Pulmonates. Ibid. 9 (1910). 
Lillie, F. R., Observations and Experiments Concerning the Elementary Phenomena o£ 
Embryonic Development in Chaetopterus. Ibid. 3 (1906). Polarity and Bilaterality of 
the Annelid Egg, Experiments, with Centrifugal Force. Biol. Bull., 16 (1909). 
Morgan, T. H., Cytological Studies on Centrifuged Eggs. /. Exp. Zool, 9 (1910). 
THE EMISSION QUANTA OF CHARACTERISTIC X-RAYS 
By David L. Webster 
JEFFERSON PHYSICAL LABORATORY. HARVARD UNIVERSITY 
Received by the Academy, January 15, 1916 
At the meeting of the American Physical Society last April, Duane 
and Hunt announced that X-rays of any given length would be excited 
as a part of the general radiation from a tungsten target only if the 
potential applied to the tube was enough to give an electron a kinetic 
energy as large as the Planck quantum of that wave length. This law 
was confirmed and extended to a potential of 100 kv by Hull. Off 
hand one would expect it to apply to characteristic rays also ; but, since 
some work of Whiddington suggests an exception here, it seemed desir- 
able to test such rays with the spectrometer. 
This was done with a rhodium target in a tube which Dr. Coolidge 
very kindly had made with one of his hot wire cathodes. The potentials 
were supplied by a storage battery of 20,160 cells, and could be meas- 
ured to about 1%. Neighboring potentials could be compared to about 
0.1%. 
