190 



MARGARET REED. 



1:50 P. M. 

 2:40 " 

 9:25 A. M. 

 9:30 " 

 10:35 " 



11=35 " 

 12:25 p - M. 

 1:40 " 



Beginning of 64-cell stage at dark pole. 



" 128 " " 



8 " " 



16 " " 



t < -22 " ' ' 



64 " " 



128 " " 



" 256 " " 



From this we see that if the delamination should occur between 

 the 32- and 64-cell periods the division must take place at this 

 period twice as fast as any other division. This of itself seemed 

 improbable. I therefore preserved series of eggs taken every 

 five minutes during the interval between the 32- and 128-cell 

 divisions and later sectioned them in order to determine how the 

 interior cells arise. 



The eggs were cut about 20 // thick and all the sections 

 drawn with the aid of a camera lucida. Each cell of the egg 

 was numbered and traced through the series of sections, in order 

 to determine not only how many cells were present but also to 

 ascertain how many came to the surface and how many were en- 

 tirely inside. Two series taken every five minutes between the 

 32- and 128-cell stages w r ere counted with the following as some 

 of the results : 



If delamination took place between the 32- and the 64-cell 

 period, sections of an egg at the 64-cell stage should show 64 

 cells coming to the surface, plus a certain number of cells inside, 

 but this, as the table shows, is not the case. 



Sections through the egg as late as the end of the 32-cell 

 stage show that all the cells are divided by cleavage-planes, 

 which appear on the surface. Sections through the eggs of 

 the series between the 32- and 64-cell stage show an increase 



