464 CHARLES RUSSELL BARDEEN 



3. Early cleavage stages 



In the toad's egg a fissure marking the first cleavage plane usu- 

 all}^ appears at ordinarj^ room temperatures in from two hours 

 and a quarter to two hours and three-quarters after fertilization. 

 When it is warm cleavage appears earlier; when it is cold, some- 

 what later. There is considerable individual variability shown 

 by different eggs of the same lot. The fissure of the second 

 cleavage plane varies in the time of its appearance from three 

 and one-quarter to six hours; the third varies from four to nine 

 hours; the fourth from five to ten hours. In the later cleavage 

 stages the variability becomes still more marked but by the twelfth 

 hour rapid cell division is usually well under way. It is during 

 the earher period of cleavage that the susceptibility of the ovum 

 to the rays becomes greatest. This is illustrated in table 5a. 



In this table are summarized the results of exposing groups from 

 two batches of eggs at successive forty-five minute intervals, 

 A-lf, A-2|, A-3i A-4, A-5, A-6, A-6f, A-7i A-Qf, A-11, and 

 E-4 and E-12, groups from three other batches for thirty min- 

 utes, C-1^, C-2, D-lf, D-6, and All, and groups from a sixth 

 batch for fifteen and thirty-five minutes, (G-6). 



The eggs exposed for thirty minutes show from a period one 

 and one-half hours after fertilization onwards an increasing sus- 

 ceptibility to the rays. In lot C-1^, 10.2 per cent of the fertilized 

 eggs developed into tadpoles and nearly half of these lived for 

 two weeks after fertilization. Discontinuance of the experiment 

 at this period makes it impossible to state how many of these 

 might have developed normally and undergone metamorphosis. 

 Comparison of this experiment with experiments C-1 and D-l|, 

 table 4, shows that after the period of 'rest' after fertilization, 

 (p. 462), susceptibility rapidly increases as the period of cleavage 

 approaches. This increased susceptibility is still more marked 

 in Experiment D-lf in which the eggs were exposed between one 

 and three-quarters and two and one-quarter hours after fertiliz- 

 ation and from which no free swimming tadpoles developed. 

 It is slightly less marked in Experiment C-2, exposure two to two 

 and one-half hours after fertilization, than in Experiment D-lf, 



