EARLY STAGES OP DEVELOPMENT OF THE BABBIT. 157 



further stretching of the outer epiblast, or it might cause its 

 rupture. 



I believe it causes the rupture of the Rauber layer ; whether 

 the several fractures between the cells are sharp, or whether, as 

 is possible, the cells are pulled apart so as to produce large 

 meshes through which the inner layer cells come to the sur- 

 face, I cannot say. If the latter case, although the Rauber 

 layer as a membrane would be obliterated, still the continuity 

 of protoplasm would not be broken ; but I think, from considera- 

 tion of all the appearances, the fractures are fairly clean. 



We now come to the question of the fate of the Rauber layer 

 cells. Kolliker says they disintegrate and disappear; Balfour 

 and Heape say they pass into the inner layer of epiblast. I 

 am fairly confident that the latter view is correct. 



If my description and hypothesis are correct, the Rauber 

 cells, as soon as they become broken up, are no longer under 

 such a high degree of tension. If they have any vitality in 

 them they will, as they grow, be able to assume other shapes 

 than " squamous.'^ They will become intermingled with the 

 cells of the inner epiblast layer. Their nuclei will always form 

 a knob, and thereby perform the part of the thin edge of a 

 wedge to the whole cell, and so on recovering what we must 

 allow was their more normal shape, lost only under pressure of 

 the fluid within, will tend to become intermingled with the cells 

 of the lower layer. No doubt some will get settled earlier than 

 others, and often upon the eighth day it is possible still to 

 detect a Rauber cell not yet accommodated. Many stages of 

 this process, I believe, are shown in my figs. 32, 33, and 34. 

 These are sections from the same specimen, which was taken 

 from a rabbit at the i53rd hour, preserved in Flemming's strong 

 osmic-aceto-chromic fluid, and stained with a weak borax car- 

 mine. In fig. 34 to the left is the thin layer of outer epiblast 

 (formerly continuous with Rauber's layer alone) (EP. 0.), 

 lying between the albumen layer {ALB.) and hypoblast {HY.). 

 To the right is the fused or fusing inner epiblast and " Rauber 

 cells " or outer layer of epiblast. The greater part is stained 

 only slightly, and contains numerous large rounded nuclei 



