84 T. H. MORGAN. 



moet peculiar and interesting type of these larvae is that in which 

 the embryo is formed in the interior of the egg instead of on the 

 surface. In other cases the material of the top of the egg ac- 

 cumulates as a solid black cap upon which the anterior end of 

 the neural plate sometimes appears. Embryos also appear in 

 which the dark cells extend to different distances over the yolk. 



The material that I used to study these embryos lacked the 

 earlier stages, and while the observations on the living eggs left 

 little doubt that the results were due to the failure of the upper 

 cells to move downwards over the yolk, yet the actual details 

 were not known. Since the question of the origin of the embryo- 

 forming material is involved in my interpretation of the results 

 it was desirable to obtain these missing stages. Dr. N. AI. 

 Stevens has kindly put up for me the necessary material for 

 studying these missing stages of Rana palnstris, the same species 

 that I formerly used. 



The eggs in the two-cell stage were put into a 0.5 per cent, 

 solution of lithium chlorid and preserved at intervals of two hours. 

 After the first two hours the eggs were in the eight-cell stage. 

 A section through one of these is represented in Fig. 26. It 

 differs little from a normal egg. 1 The segmentation cavity is 

 somewhat smaller than that of the normal egg at this time. Two 

 hours later the egg had reached the condition shown in Fig. 27. 

 The inpulling of the cells of the upper hemisphere has begun. 

 The segmentation cavity is noticeably small. Two hours later. 

 Fig. 28, the divisions have gone further, and the upper cells now 

 form almost a solid cap around the upper pole. A very small 

 segmentation cavity, 5", is present. Two hours later, Fig. 29, the 

 smaller cells at the top still remain in place. A flattened seg- 

 mentation cavity is present, but much reduced as compared with 

 that of the normal egg. During the next four hours the changes 

 are not marked. The small cells remain at the top and the seg- 

 mentation cavity remains small. Two hours after this, /. c., six 

 hours after the last stage figured, the eggs are in the condition 

 represented in Fig, 30. The roof of the segmentation cavity is 

 very thick and the small cells still remain in the upper hemi- 

 sphere. Four hours later, Fig. 31, the material at the top of the 

 1 In preserving the lower blastomeres became separated in the egg. 



