308 | Mr. J. F. Gaskell. | [Nov. 11, 
exposed. This is not, therefore, taken as any evidence of any stimulating 
effect on their growth. 
The question then arose as to whether the effect was cumulative or recovery 
was complete. A further experiment was undertaken, and the treatment, 
was stopped on the twelfth day. The results are shown in Table III. 
Table III. 
| Length of Distance ; Average 
daily from Results. growth 
exposure. | anticathode. in days. 
; | 
| 20 minutes 15 cm.) Vol othe, ithe 1 Oth ie thea alot, see 9 
| 15 Bs 15.5, | 2 hatched; 1, 19th; 1, 4th; 2 sterile .................. 20 
10 % iisy 3. | Schatchedss 63 sterile <1). taea.geememes tout ee eee 21 
5 i 15 ,, 1G Matohed \asa.cctbee tons woses ccuechee eet e  e ee 21 
In this experiment, therefore, 20 minutes’ exposure killed on or before the 
twelfth day, that is to say, death occurred while exposure was still going on. 
The average development of nine days agrees fairly well with Table II, in 
which a 20 minutes’ dose gave development to six days. The fact that a 
different X-ray tube was being used may also, to some extent, account for 
the difference. 
Those given a 15 minutes’ exposure were able to hatch out, though in the 
previous experiments this did not occur. These embryos had not yet reached 
the critical stage of development at which an X-ray dose of 15 minutes takes. 
effect ; they therefore survived, and were able to hatch out at the usual time. 
The embryo which only reached the fourth day of development is neglected, 
as this was probably accidental. 
Those exposed for 10 minutes and 5 minutes hatched out as in the previous 
experiments at the usual 21 days. The evidence from this experiment there- 
fore tends to show that the effect is not a cumulative one, but that there is a 
critical point at which a certain exposure stops development. 
Further experiments were intended to support this view. Eggs were to 
be incubated up to a certain day and then given a single exposure of length 
differing in the case of each group. In such a series the particular length of 
exposure which would just prevent further development ought to agree with 
that obtained from Table II, if the effect is non-cumulative. All those 
receiving a smaller exposure should hatch out. It has, however, at present. 
not been possible to continue these experiments, so that this evidence is as. 
yet lacking. Additional facts supporting the non-cumulative view have been 
obtained from embryos opened at various stages of development while under- 
