SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 467 



more becomes very high. (Experiments A-3j and A-4.) In 

 these experiments most of the ova stopped developing very 

 early in the period of gastrulation. A few show irregular gas- 

 trulation with a large protruding yolk plug. In Experiment E-4 

 eggs in about the same stage of development as those in Experi- 

 ment Al-4 (2-8 cell stage but chiefly 2 cell) were exposed for 

 forty-five minutes with a result closely similar to that found in 

 the latter experiment. 



During the eight and sixteen cell stages the susceptibility re- 

 mains very high, as may be seen by comparing Experiments A-5 

 D-6, A-6 and G-6 with the experiments just described. It will 

 be noted that even an half-hour exposure is sufficient to inhibit 

 development beyond mere gastrulation and to stop the process 

 of gastrulation at a very early period except in about 10 per cent 

 of specimens in one experiment, D-6. After a fifteen minute 

 exposure, G-6, but 15.3 per cent developed into definite larvae 

 and these were abnormal and died early. 



After the sixteen cell stage the susceptibility gradually becomes 

 less as shown in Experiments A-6f, A-7|, A-9|, and A-11. In 

 Experiment A-6f , (exposure from six and three-quarters to seven 

 and one-half hours after fertilization,) 5 per cent of the eggs 

 exhibited an irregular gastrulation with protruding yolk; in 

 Experiment A-7^, (exposure from seven and one-half to eight 

 and one-quarter hours after fertilization,) 10.8 per cent acquired 

 a closed blastopore before ceasing to develop; in Experiment 

 A-9|, (exposure from nine and one-half to ten and one-fourth 

 hours after fertilization) 15.9 per cent developed into pig- 

 ment covered larvae of which nearly half showed heads and 

 tails; in Experiment All, (exposure from eleven to eleven and 

 three-quarter hours after fertilization,) 57.9 per cent devel- 

 oped into pigment covered larvae half of which showed heads 

 and tails. In Experiment A-95, 21 . 7 per cent of the specimens 

 were of spina bifida type. After the period represented by this 

 experiment exposure to the rays did not cause spina bifida 

 although in one instance it caused an egg to cease development 

 early in gastrulation. The eggs in Experiment E-12 were unu- 

 sually susceptible compared with those in Experiment A-11. 



THE AMERICAN JOURNAL OF ANATOMT, VOL. 11, NO. 4 



