SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 455 



do not manifest themselves except oecasionalh' before the later 

 stages of larval differentiation.^ The injurious effects of exposing 

 the ova before fertilization make themselves manifest early in 

 development in a large number of the specimens. In both sets 

 of experiments hemi-larvse are occasional!}^ produced (figs. 4 

 and 5), while in specimens exposed to the X ravs after fertiliz- 

 ation no abnormal forms of this type were found. In one instance 

 in a lot of eggs exposed for three-quarters of an hour after the 

 first cleavage stage had begun one specimen exhibited bilateral 

 assymetry, but it did not on careful examination appear to be a 

 true hemi-larva. In spina bifida specimens produced by salt 

 solutions the abnormal forms are nearly always symmetrical or 

 nearly symmetrical. In an examination of a large number of 

 specimens of this kind I have found no hemi-larvse. It would, 

 therefore, appear that occasionally one germ cell nucleus or the 

 other may at least predominate in influence in one of the first 

 two cleavage cells destined to form a lateral half of the body, and 

 that if this sex cell nucleus is sufficiently injured no development 

 of the blastomere takes place. 



If it be true that a sex cell nucleus can be so injured by exposure 

 to the rays that it becomes incapable of initiating or properly 

 governing development even in the earliest stages, we have an 

 explanation of the greater number of early abnormalities seen 

 after exposing the unfertilized ova. In such eggs we may assume 

 that if the nucleus has been sufficiently injured development must 

 be guided largely by the male sex elements. These probably are 

 at first not intimately adjusted to the demands of the protoplasm 

 of the egg and hence development appears abnormal at an early 

 stage. In the later stages it appears that the male and female 

 sex elements are by no means evenly distributed in the differen- 

 tiating tissues. The tissues in which the injured germ plasm 

 predominates are those which first show abnormalities and these 

 differ in different specimens. In the few specimens which sur- 

 vive it is probable that every tissue gets some normal germinal 

 elements derived from the uninjured germ cell. 



^The experiments of Hertwig, 1910, however, show that severe prolonged 

 radium irradiation can give rise to disturbances manifest earlier. 



