SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 



487 



Ihr. lom. 



expos. 



sperm 



1 hr 



Bxpos. 



Fertilization 

 Cleavage 



Dastrulation 



Larval 

 differentiation 



Differentiation 



tadpole 



Growth 

 tadpole 



metamorphosis 



Diagram A 



are represented as of equal proportions in the diagram. It is 

 assumed that all eggs fertilized by the non-irradiated sperm devel- 

 op normally and imdergo metamorphosis if not exposed to the 

 rays. The course of development of the eggs fertilized bj'- irra- 

 diated sperm or directly irradiated is represented by shading and 

 deviations from the normal are represented by carrying the shad- 

 ing to the right of the column representing the normal course of 

 development. 



Thus some of the eggs fertilized by sperm irradiated one hour 

 appear abnormal during gastrulation but most first appear abnor- 

 mal during larval differentiation and before hatching. Few are 

 differentiated into tadpoles capable of much development. 



In ova exposed for one hour before fertilization the abnormali- 

 ties appear early in greater numbers than in eggs fertilized by 

 irradiated sperm, but about the same relative number become well 

 developed tadpoles. Eggs exposed during fertilization for forty- 

 five minutes show many abnormal forms during the later cleavage 

 stages and during gastrulation. Few are definitely differentiated 

 into larvae. 



Of eggs exposed during the early cleavage stages for forty-five 

 minutes few undergo gastrulation. 



Eggs exposed early in gastrulation for forty-five minutes prac- 

 tically all develop normally but if given two successive forty-five 



