1910.| The Action of X-Rays on the Developing Chick. 307 
minutes’ duration, in order to settle this point, have not been given in these 
experiments. 
The effect of this diminution of mitotic activity was then tried on the 
development of the embryo. Daily exposures of varying length were given 
to eggs which were incubated for the full 21 days. These were opened on 
the twenty-third day if hatching had not taken place. The results of various 
experiments are grouped together in Table II. In all these experiments the 
controls hatched out at the proper time, with the exception of three, which 
were apparently sterile. 
Table II. 
Length of Distance Average 
daily from anti- | Results. growth in 
exposure. cathode. days. 
2 hours 15 cm. Gre COS Wit eT OFOTOMU Me aut saecesccsadseutoc cotiecasaseee csieet 0 
30 minutes 2 ZeSLAL ARVO. MON OTOWED: 4siets sas cers eqeneen sone: ssf nee a08 1°25 
| 30 : 201%, SuOvMNGay pe OrGs) bo NOsCLOW tL! sceya.tecanccts cn teseacee 4 °5 
0) eae 20 ,, 1, 10th day; 1, 7th; 1, 6th; 1, 5th; 1, 8rd ......... 6-2 
Tete cs toa 1, 14th day; 1, 13th; 1, 9th; 1 sterile..............0. 12 
10 4 aa aphatelWediss Wy Otis UNG) os sccncescacsddsedonesscecss 19 °5 
10 Py 20%. 2 hatched ; 1 opened on 17th day; 2 sterile............ 21 
5 3 oes SACC MEO s Le COLMA cesnstaysneeiieacnen es declatsite.vesivels «ci 21 
In the calculations the sterile eggs and the one in the last series which 
only reached the eighth day’s development are neglected, this latter being 
decomposed. 
The variations in the individual groups may be partly due to the fact that 
each group of eggs was exposed together ina box; the eggs were not, therefore, 
all equidistant from the anti-cathode. Also the X-ray tube did not give a 
constant dose throughout the experiments. Attempts were made to measure 
the dose by means of Sabaraud’s pastilles, but the results were unsatisfactory, 
especially in the case of the short exposures. 
The table shows that a daily exposure of a certain length stops further 
development of the embryo at a certain stage. The stage of development 
bears an inverse ratio to the exposure given. The critical dose, which 
renders the hatching of the chick doubtful, is a daily exposure of 
10 minutes at 12 cm. distance in this series of experiments. Those exposed 
to a smaller daily dose all successfully hatched out. Those given a daily 
five minutes’ exposure hatched out a few hours earlier than their controls 
but this was probably due to their shorter stay outside the incubator and 
their consequent shorter period for cooling. The controls were always left 
out 20 minutes at least, while the others only stayed out while being 
VOL. LXXXIII—B. 2A 
