480 CHARLES RUSSELL BARDEEN 



the neural canal were abnormally dilated. In another experi- 

 ment out of 111 fertilized eggs exposed at the beginning of gas- 

 trulation for two and one-half hours 32 . 4 died early in the period 

 of larval differentiation and the rest before hatching. In a third 

 experiment all of a lot of seventy-five frogs' eggs exposed at the 

 time of closure of the blastopore were abnormal in form by the period 

 of hatching and died just before or after this period. 



Godlewski's interesting studies on the relative proportions of 

 nuclear to cytoplasmic material during the early development of 

 the eggs of sea urchins have already been cited, (p. 470.) Dur- 

 ing the blastula period he finds that the nuclei increase rapidly 

 in number but at the same time decrease in size so that the pro- 

 portion of nuclear to cytoplasmic material is not greatly increased. 



It is essentially a period of distribution of nuclear material, of 

 increase of nuclear surface but of slow increase of nuclear substance. 

 If similar conditions prevail in amphibian eggs the greater sus- 

 ceptibility during the earlier cleavage stages as compared with 

 the blastula stages may probably be ascribed to a greater sus- 

 ceptibility of cells when the production of nuclear material is 

 rapid than when it is slow. This likewise would apply to the 

 gastrula and early larval stages. In the sea urchin Godlewski 

 finds during the gastrula and pluteus stages that the nuclei 

 gradually multiply in number but remain of about the same size. 

 The production of new nuclei material is far less rapid than in 

 the early cleavage stages. 



Ruffini ('08) has shown that in amphibian eggs gastrulation 

 depends largely on cell migration and on cell secretion. The 

 neural tube is likewise formed by processes of cell migration 

 and cell secretion, the latter due to the periectoderm cells which 

 come to line the medullary tube. Osmosis likewise plays a 

 part in the swelling of closed cavaties. There is little evi- 

 dence that exposure to the rays affects these various proc- 

 esses directly. Exposure during the early cleavage stages may 

 prevent gastrulation or render gastrulation very abnormal by 

 preventing the formation of the normal cells on which the proc- 

 ess depends. But if these cells are once formed gastrulation 

 usually goes on well in spite of severe exposure. The effects 



