THE FORMATION OF THE INTERIOR CELLS IN 

 THE SEGMENTATION OF THE FROG'S EGG. 



MARGARET REED. 



Sections through the egg of the frog in the early stages of 

 segmentation show that the cleavage planes extend from the 

 surface inwards forming but one layer of cells. Later cleavage 

 stages, however, show certain cells entirely within the interior of 

 the egg, so that the cells of the dark pole appear to be arranged 

 in two layers about the segmentation cavity. 



Former observers supposed this appearance to be caused by a 

 cleavage plane coming in parallel to the surface of the egg 

 (delamination) probably between the 32- and 64-cell period. 



Following Professor Morgan's suggestion, I attempted to 

 determine whether a delamination really occurs or whether the 

 interior cells are produced in some other way. 



If the delamination occurs between the 32- and 64-cell stages 

 we should expect to find the -interval between these cleavage 

 stages, as seen on the surface, longer than that between any two 

 preceding divisions, because somewhere during this time, the 

 cells would have divided by planes, which do not appear on the 

 surface. 



In order to test this point, I made a number of observations 

 on the living egg, noticing the times of the surface divisions. I 

 found no marked difference in the length of the interval between 

 any two divisions, but the cleavage took place in a regular 

 rhythm. There was usually a period of about one hour from the 

 first appearance of segmentation in the cells of the dark pole 

 until the next cleavage again began to appear there. This is 

 shown in the following table. 



9:25 A. M. Beginning of 2-cell stage at dark pole. 



10:15 " " 4 " " 



11:15 " " 8 " 



12:10 P. M. " 16 " " 



12:55 " 3 2 " " 



189 



