REGENERATION 453 



had been indicated from earlier investigations by the famiUar techniques, 

 it is being more effectively demonstrated by the technique of irradiation. 

 Without expecting too much from further studies upon regeneration 

 by means of irradiation, it may be hoped that a method of experimenta- 

 tion capable of destroying certain types of cells, as by an operation of far 

 greater deUcacy than could be accomplished by the finest microdissection, 

 will jdeld many important results in this field. There is no other method 

 by which more profound internal changes can be effected, in some 

 instances without ultimate destruction of the organism. Since radium 

 and X-rays affect primarily the cells with embryonic potentialities, one 

 can remove these cells, as though by dissection, and hope to discover the 

 kind of organism which then exists. That the differential in sensitivity 

 is even finer than this appears from the way in which the cells in the cap 

 at the anterior end of a regenerating planarian lose their extreme sensi- 

 tivity as differentiation proceeds in the same manner as differentiating 

 cells during embryonic and later development. As the converse of this 

 one would expect an increase of sensitivity in any case of dedifferentiation 

 that was clearly demonstrable. The finer differential is seen again in the 

 differing effects as the time relationship between exposure and operation 

 is shifted; and in the case where X-rays inhibit cartilage differentiation 

 in the amputated limb of Amhly stoma during a period when differentia- 

 tion of this tissue is proceeding in the opposite limb w^iich was left intact. 

 There is also the possibility of distinguishing critical periods in differen- 

 tiation as suggested by some of the observations on planarians and on 

 Amhlystoma. The vexed questions of the potencies and of the origin of 

 formative cells along with the problem of contrasting powers of regenera- 

 tion can be attacked as by no other technique ; and it should be possible 

 to separate more effectively the histological from what we have called 

 the organismal factors in regeneration. A study of such problems 

 becomes a study of the general problem of cell differentiation and deter- 

 minism. In all this it is highly desirable that we should learn more 

 concerning the fundamental effects of the radium and X-rays upon 

 protoplasm by study of material more favorable for the purpose than is the 

 regenerating tissue of most animals. 



REFERENCES 



1. Alberti, W., and G. Politzer. tJber den Einfluss der Rontgenstrahlen auf die 

 Zellteilung. Arch. Mikrosk. Anat. und Entwicklungsmech. (Roux) 100 : 83-109. 

 1924. 



2. Alberti, W., and G. Politzer. tTber den Einfluss der Rontgenstrahlung auf die 

 Zellteilung. II. Mitteilung. Arch. Mikrosk. Anat. und Entwicklungsmech. 

 (Roux) 103: 284-307. 1924. 



3. Bardeen, C. R., and F. H. Baetjer. The inhibitive action of the Roentgen rays 

 on regeneration in planarians. Jour. Exp. Zool. 1: 191-195. 1904. 



4. Bartsch, O. Die Histiogenesis der Planarienregenerate. Arch. Mikrosk. Anat. 

 und Entwicklungsmech. (Roux) 99: 187-221. 1923. 



