1910.] The Action of X-Rays on the Developing Chick. 309 
going daily exposures. If alive, they were always at the correct stage of 
development; none gave evidence of any retardation. The fact that the 
X-rayed embryos which received small doses hatched at the usual time also 
implies that the recovery from the X-ray effect is complete for each exposure. 
The hatched chicks were kept alive to watch their further development. 
Unluckily, the first to hatch out under X-ray treatment did so in the late 
autumn, and the conditions were unfavourable for their proper development ; 
but nevertheless no difference could be observed between them and their 
controls, They were killed and examined at various stages, but no differences 
were discovered in the development of any of their organs. This is in accord 
with the microscopical evidence given earlier in this paper: the X-ray action 
is uniform on all tissues. 
Particular attention was paid to the generative organs, but neither macro- 
scopically nor microscopically did they differ from the controls in any way. 
The chicks obtained from the experiment in Table III are at present being 
brought up with the intention of testing their breeding capabilities, to see if 
any action has occurred on their generative cells. They are at present all 
normal in appearance. 
The meaning of these experiments, and the probable action of X-rays on 
growing tissues, may now be considered. It has been shown by Minot,” by 
actually counting the number of mitoses per 1000 cells in the various tissues 
of embryos of different ages, that throughout embryonic life a rapid diminution 
of mitotic activity is going on. He calls the figures obtained the mitotic 
index for that particular tissue. 
This mitotic index varies at any particular stage of development according 
to the rapidity of growth of the particular organ at that moment, but this 
variation is not sufficient to obscure the general diminution of the index 
which is taking place. The effect of X-rays is to bring about rapidly 
a progressive lowering of this mitotic index in all the tissues of the developing 
embryo. This lowering is not permanent, and is completely recovered from 
so long as it has not been of too great a degree. If, however, the exposure 
has been of a certain critical length, recovery no longer takes place, and the 
embryo dies. This critical dose depends upon the stage of development 
reached by the embryo, that is to say, upon the value of the general mitotic 
index at that stage; the larger the mitotic index is—that is to say, the 
greater the reproductive activity—the larger is the X-ray dose required to 
prevent further development. 
The X-rays are, therefore, directly antagonistic to the reproductive activity 
* “The Problem of Age, Growth, and Death,” ‘Popular Science Monthly,’ 1907, 
vol. 71, p. 510. 
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