520 RADIATION BIOLOGY 



spatial limitation in joining appears difficult to reconcile with the idea 

 that the chromosomes are arranged end to end in single file within the 

 spermatozoon, as had once been inferred (Gowen and Gay, 1933) from 

 the approximate correspondence between the total length of the con- 

 densed chromosomes of a haploid set and the length of a sperm head and 

 as Cooper (1952) has reported on the basis of the cytological observation 

 of an individual spermatozoon. For, if this were true, the broken ends 

 of two different chromosomes could only be near enough together to form 

 a translocation when the breaks were both near the original free ends (a 

 case which is exceptional for observed translocations), and large deletions 

 and inversions could never be produced. 



Nevertheless, the linear relation found with neutrons would demand 

 spatial limitation of both breaks and unions, unless the linearity was in 

 this case a "synthetic" resultant of two antagonistically acting factors. 

 These presumably would be (1) a primary relation of frequency to dose 

 similar to that found with X rays and (2), superimposed on factor (1), 

 some contrary influence. With increasing dose, factor (2) would either 

 make chromosomes ever less susceptible to breakage or to exchange 

 union or, more likely, would result in a selection of offspring from classes 

 of sperm which had naturally been less susceptible. As has been noted 

 earlier in connection with X-ray results, Llining's work (1952a, b, c, d) 

 shows the existence of such a selective factor when the sperm are of 

 mixed stages of maturity or when the males providing them are of mixed 

 ages. The deviations in the data, suggesting that the translocation fre- 

 quencies may with higher doses of neutrons fall even below those expected 

 on linearity, and the similar results obtained by Muller (unpublished 

 data) for chromosome losses produced by neutron treatment, indicate 

 that such an influence was in fact acting to some degree in this work. 

 Hence a decisive answer cannot be given to this question before additional 

 data are obtained with material more exactly uniform in regard to age of 

 males and stage of their contained sperm. 



Although a truly linear relation for the production of gross structural 

 changes by neutrons applied to spermatozoa would prove the principle of 

 spatial limitation in breakage, nevertheless, a relation like that found for 

 X rays would not disprove it, since such a result could be explained as an 

 effect of the unions rather than the breaks having a considerable spatial 

 range. However, in view of the margin of doubt about the meaning of 

 the apparently linear neutron results actually obtained, it is fortunate 

 that there is a series of experiments of another kind on the genetic effects 

 of neutrons on this material, which definitely show the existence of a fine- 

 scale spatial limitation in mutagenesis by ionizing radiation. This work 

 is concerned with the production of point mutations. 



Following Nagai and Locher's (1937) demonstration, under Alten- 

 burg's direction, of the production of sex-linked lethals by neutron treat- 



