SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 479 



ing and after gastrulation the alterations are not regionally local- 

 ized although much more marked in some organs than in others. 



Of the various organ systems the central nervous system, 

 including the optic stalk and retina, is the most regularly affected. 

 The neural tube is usually dilated and thin walled. It seems 

 quite evident that the neuroblasts lose the power of normal 

 differentiation and, failing to differentiate in a normal manner, 

 they finally undergo retrograde metamorphosis giving rise to 

 irregular protopoasmic masses frequently pigmented. The earlier 

 the time or the more severe the exposure the earlier do the dis- 

 turbances in the neuroblasts become manifest. 



The vascular system is likewise usually markedly affected in 

 specimens exposed during gastrulation. The heart and blood 

 vessels are, as a rule, very rudimentary and the blood corpuscles 

 seem to be much reduced in number or missing. External gills 

 usually appear in these specimens although not as a rule in 

 specimens severely exposed during the earlier stages. 



The alimentary canal in the specimens severely exposed during 

 gastrulation seldom shows much differentiation of coils and the 

 pancreas and liver are rudimentary. The mouth in most speci- 

 mens is patent. The pharynx is poorly differentiated. 



The pericardial and peritoneal cavaties are usually greatly 

 dilated. 



The pronephric tubules are sometimes abnormally dilated. 

 The sex cells seem fairly normal. 



The chorda is usually relatively normal but may appear dis- 

 tended. 



The myotomes and the muscles of the head are as a rule poorly 

 differentiated. 



Frog eggs show, at times at least, a greater susceptibility to 

 the X rays than toad eggs during the gastrulation period. In one 

 experiment in which frogs' eggs with small blastopores were 

 exposed fifty minutes to the rays practically all the larvae at the 

 time of hatching showed marked abnormalities and none sur- 

 vived this period. The chief abnormalities were seen in the ali- 

 mentary canal, the central nervous system and the vascular 

 system. In a considerable number of specimens both the gut and 



