SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 445 



relatively well developed except in those forms in which the whole 

 head is very profoundly affected. 



Skeletal tissues. Except in those specimens in which larval 

 differentiation is interrupted at a very early period a distinet 

 mesoblast with visceral and parietal layers is usually differentiated 

 in the region of the trunk. In very many specimens the perito- 

 neal, like the pericardial cavity, subsequently becomes abnormally 

 dilated. In those specimens which reach a relatively advanced 

 stage of development the loose connective tissue of the head and 

 body wall frequently appears much distended with fluid and 

 abnormally great in extent compared with the more definitive 

 organs. The chorda dorsalis in those specimens which reach an 

 advanced stage of larval development appears to be relatively 

 normal in form, although frequently it may be in places assym- 

 etrical, dilated, or small and defective. 



Cartilages and other definitive skeletal structures although 

 generally somewhat abnormal in form, are differentiated in a 

 relatively normal manner in those specimens which reach the 

 stage of skeletal differentiation. Most specimens die before 

 auditory capsules are produced. 



5. Tadpole stage 



In all of the successful experiments with irradiated sperm 

 some of the fertilized ova developed into tadpoles fairly normal 

 in external form, with operculum, rounded belly, and membranous 

 tail. Most of these tadpoles, however, failed to grow and expand 

 like normal tadpoles, and very few reached and successfully passed 

 through the period of metamorphosis. The majority soon exhib- 

 ited externally visible defects of one sort or another, and within 

 two or three weeks after the fertilization of the ova, died off. 

 In table 1 these tadpoles are classed as 'defective'. From this 

 table it will be seen that they constitute from 1.3 per cent to 

 26 per cent of the total number of eggs fertilized. The number 

 of specimens undergoing apparently normal development, al- 

 though not in all cases followed to metamorphosis was 8.3 per 

 cent in one experiment in which the sperm of the frog was exposed 



