310 The Action of X-Rays on the Developing Chick. 
of the cell, and act in such a way as to lower, and finally to destroy, its power 
of reproduction. 
I am greatly indebted to Prof. Griinbaum for the suggestion of this 
research, for the use of the apparatus, and for his advice and encouragement. 
For some months previously he had been working at this subject, and had 
been assisted by a grant from the Grant Committee of the Royal Society. I 
am fully aware that this paper is of the nature of a preliminary communi- 
cation, and that the investigation is by no means complete. As, however, I 
am unlikely to be able to continue it at present, I have thought it best (with 
the concurrence of Prof. Griinbaum) to publish it in its present unfinished 
condition rather than to wait for a further opportunity to complete the 
investigation. 
Summary. 
1. The action of X-rays is to lower the mitotic activity of growing tissues. 
2. If this diminution is not too great, complete recovery occurs. 
3. If the diminution is above a certain degree, recovery does not take 
place. 
4, The critical dose, which just prevents recovery, varies with the stage of 
development of the embryo, decreasing as the mitotic index decreases. 
5. No other macroscopic or microscopic changes have been observed to 
take place under the action of X-rays, either in the proportions or structure 
of the various tissues. 
