Abnormal Development of Toad Ova 3 



germinate normally at first and then show a temporary or per- 

 manent inhibition of growth. Koernicke found in studying the 

 tissues of exposed plants that it is the nuclei that seem especially 

 affected by the rays. There is no visible direct injury to the 

 cytoplasm. The effect on the nuclei is proportional to the length 

 of exposure. The nuclei of the vegetative cells are more resistant 

 than the pollen mother-cells. Owing to injury to the nuclei of 

 the pollen mother-cells the pollen cells may be abnormal in 

 appearance but this is due tp the action of the injured nuclei on the 

 cytoplasm. 



In the pollen mother-cells twenty-four hours after five hours' 

 exposure to radium rays the nuclear threads during mitosis fell into 

 small double segments which were much smaller and more numer- 

 ous than normal for Lilium martagon (the species studied). 

 Division of the chromosomes took place somewhat in the normal 

 manner but the daughter chromosomes seldom passed simul- 

 taneously toward the poles. Occasionally two or three daughter 

 nuclei were formed on each side of the equator. If the pollen 

 mother-cells shortly before the diakinesis of the nuclei were 

 exposed for twenty-four hours, they showed a day later a clumping 

 of the chromosomes at the center of the nuclear cavity. The 

 spindle figure was strongly developed. Sometimes the spindle 

 poles seemed split. 



Exposure to radium rays up to ten hours seemed to have little 

 effect on the daughter cells of the pollen mother-cells, but a one to 

 three days' exposure had a marked effect. The nuclei were brought 

 into an abnormal stage which partially resembled a resting stage. 

 The nuclei of the tetrads arising from exposed cells were also very 

 abnormal. 



The effects of the Roentgen and radium rays on a protozoa have 

 been studied by Schaudinn, Joseph and Prowazek, Zuelzer, and 

 others. Schaudinn^ showed that individuals of several species of 

 protozoa may be killed by exposure to the Roentgen rays for a 

 few hours, while others are not thus susceptible. Joseph and 

 Prowazek^" found that Paramecia and Daphnia show a negatived 



'Schaudinn: Archiv. f. die gesammte Physiologic, Ixxvii, p. 29, 1899. 

 ^"Joseph and Prowazek: Zeitschr. fiir allg. Physiol., Bd. i, 1902. 



