SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 469 



are severely affected the less affected cells may not only recover 

 themselves but also be enabled to overcome to some extent the 

 injurious effects of the more affected cells of the organism. 

 Whether this explanation alone would account for the decrease 

 in susceptibility in the later cleavage stages is open to question. 

 It is not improbable that as differentiation proceeds the individual 

 cells become relatively more stable as they decrease in potential- 

 ity of differentiation. 



The susceptibility during the early cleavage stages is greater 

 than that exhibited by either of the sex cells before fertilization 

 or by the fertilized ovum during fertilization. The effects of 

 exposure are not, however, exhibited immediately, even during 

 the periods of greatest susceptibility. Cleavage usually goes on 

 until the period of gastrulation is approached. As a rule, cleav- 

 age is more or less incomplete in the yolk laden protoplasm of the 

 vegetable poll. After forty-five minutes exposure to powerful 

 X rays during the early cleavage period gastrulation is either 

 inhibited in its earliest stages or begins in an irregular manner 

 and then stops with a mass of yolk projecting through a very large 

 blastopore. After thirty minutes exposure the blastopore may 

 close but development then stops. During cleavage after expo- 

 sure the cleavage cavity is not infrequently abnormally small. On 

 the other hand, it may be abnormally dilated. Spina bifida 

 specimens seem to arise usually from forms in which the cleavage 

 cavity has been abnormally small. When the cleavage cavity is 

 well developed but gastrulation incomplete in that the yolk is 

 not completely covered, the yolk mass in the larval stage protrudes 

 through the anal region instead of the back. 



In frogs' eggs the susceptibility is likewise greatest during the 

 early cleavage stages but it appears to decline less rapidly than 

 in toads' eggs." The cause of this is obscure. 



Hertwig and others^ have shown that eggs vary in suscepti- 

 bility to changes in temperature and other external conditions 



^For the literature see Handbuch der ver^leichen und experimentellen Ent- 

 wicklungsgeschichte. 



