SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 439 



after the blastopore closed. In some of these specimens the arch- 

 enteron was very abnormally distended, in others it was rudi- 

 mentary in character. A neural plate was well marked in some 

 of the specimens. In some rudimentary traces of a notochord 

 were found. In all the other experiments the specimens in 

 which the gastrulation was internally abnormal went on to some- 

 what further development so that specimens with a more or less 

 distinctly larval form were produced. 



4. Period of larval differentiation 



(a) Early stages, (up to the appearance of the rudiments of the 

 head and tail). In most of the experiments some of the specimens 

 ceased to develop so early in the larval period or were so abnormal 

 that the rudiments of the head and tail were not very distinct. 

 In those experiments in which definite data were obtained concern- 

 ing the proportionate number of forms of this character the per- 

 centage varied from 3 . 7 to 9 . 5 except in the atypical Experiment 

 2, in which the percentage reached 20 . 8. Examination of these 

 rudimentary larvae revealed the fact that in most instances the ab- 

 normalities must have begun to appear within the body during the 

 gastrulation period. The extent of structural differentiation 

 varied greatly. As a rule the neural tube rudiment was a some- 

 what irregular mass of cells without a trace of lumen. The ali- 

 mentary canal in some specimens was fairly well developed, in 

 others it was either greatly dilated or very rudimentary. In 

 some specimens in which other structures were but little differen- 

 tiated some of the myotomes were fairly distinct. The noto- 

 chord was at times somewhat differentiated, at other times not 

 distinctly to be made out. Irregular outgrowths in some speci- 

 mens indicated a head or tail (fig. 13). In one specimen a large 

 ' cleavage ' cavity appeared at one side of the body. 



(b) Later stages, (from the period when the rudiments of 

 the head and tail are distinct up to the time when normally the 

 definitive tadpole form is assumed.) Specimens which at the 

 beginning of the larval differentiation appear nearly normal 

 frequently begin to appear abnormal during the formation of 



