SUSCEPTIBILITY OF AMPHIBIAN OVA TO X-RAYS 471 



the toad's egg there is a rapid production of chromatin preceding 

 the first cleavage.^ 



In this connection I may refer to an interesting experiment on 

 the action of concentrated sugar solution on frogs' eggs. I find 

 that if freshly fertilized eggs frogs' are placed in a six to eight per 

 cent sugar solution and are left there from two to eight hours and 

 then transferred to water all die within a few days. Very few eggs 

 develop past the neural groove stage. On the other hand, if 

 left in the same solution from twenty to thirty hours, and then 

 transferred to water, many lived to pass through a normal 

 development. This experiment indicates that a sudden change of 

 condition during the period when the chromatin is being most 

 rapidly manufactured much more seriously interferes with the 

 normal course of development than if development during this 

 period is allowed to proceed under conditions far from normal. 



For refined cytological studies the eggs and larvse of anura are 

 not well adapted and for this reason no prolonged and detailed 

 examination of the cells in the specimens obtained from the exper- 

 iments have been attempted. In the abnormal larvse many evi- 

 dences of direct nuclear division are to be found, especially in 

 the epidermis. On the other hand, occasional apparently normal 

 mitotic figures are to be seen in all of the tissues. Giant nuclei 

 are not infrequent, especially in the mesodomic tissues. In the 

 cells of the central nervous system nuclear abnormalities are 

 especially apt to be well marked. An irregular clumping of large 

 masses of chromatin within the nucleus is a common occurrence. 

 In specimens exposed during early cleavage and preserved imme- 

 diately after exposure a nucleus within a definite membrane within 

 the cleavage spindle may sometimes be seen. It would appear 

 as if the nuclei in mitosis were forced into a resting stage within 

 the spindle by the X rays. 



^Hertwig, 1910, in ascribing the great susceptibility of fertilized ova to the 

 irradiation of both the male and the female pronucleus has failed to give suf- 

 ficient weight to the increase of susceptibility during the early cleavage stages. 



