ORGAN-FORMING MATERIALS IN FROGS EMBRYO. I2J 



by the upper ends of the lower four cells. The great thickness 

 of the upper four cells that form the roof of the blastoccel is 

 especially to be noted. The next section, Fig. 2, is a section 

 through a i6-cell stage. The same relations noted for the 

 last stage hold here also. The next three figures are from 

 another set of eggs. Fig. 3, A, is a 54-cell stage with six 

 of the cells inside. The cells of the roof have already begun to 

 move out towards the sides, so that the middle part of the roof 

 is much thinner than that of the sides. The next figure, Fig. 3, 

 B, is from an egg containing sixty-four cells, of which fifteen are 

 inside. It shows changes similar to those of the last figure. 

 Fig. 3, C, is through an egg with 105 cells, of which twenty-six 

 are inside. The shifting of the cells derived from the upper four 

 cells is still more apparent. At this time the top of the roof of 

 the segmentation cavity is much reduced in thickness, while the 

 sides are still quite thick. In the last three figures, the seg- 

 mentation cavity is very irregular in shape. At first it is deeper 

 in a vertical plane, but in the last stage it is broadening out in a 

 horizontal plane. The last section, Fig. 4, is from an egg belong- 

 ing to the same set as Figs. I and 2. The following six figures, 

 Figs. 5 to 11, belong to still another series. 



The fluid in the segmentation cavity is formed by the sur- 

 rounding cells, and it may be supposed that the thinning out 

 of the roof of the blastoccel is due to loss of fluid, but as the 

 egg as a whole slowly increases in size during this time, the loss 

 of fluid to the blastoccel must be made good by the absorption 

 of water from outside. During the following stages, as seen in 

 Figs. 5-9, the roof becomes very much thinner, as shown strik- 

 ingly on comparing the sections of the roof at different stages, 

 as seen in Figs. 11-15. ^ will also be noticed on comparing 

 sections of the earlier and the later segmentation stages, that the 

 side-walls of the segmentation cavity are at all stages very thick 

 compared with the upper wall, and also that the smaller cells of 

 this region are being slowly carried down at the sides of the egg. 

 They furnish the material out of which the "embryo" is later 

 formed. In Fig. 10 the first indications of the dorsal lip are 

 present, as seen to the left. At this stage the smaller cells have 

 pushed down below the equator of the egg on the side at which 



