142__ 



The first three cleavage planes lie in the three dimensions of space, 

 and the eight cells are all equal. At the fourth cleavage period four 

 rnicromeres are cut off at one pole (by a plane of division parallel to 

 the third), and the four cells at the opposite pole divide equally and 

 radially. 



As seen in Fig. 1 b, the first four nuclei of the four cell stage lie 

 towards one pole of the egg. At the eight cell stage this excentricity 

 is less obvious. Professor Whitman has determined the important 

 point that when the rnicromeres form at the sixteen cell stage, they 

 always appear at the pole opposite to that at which the early nucleus 

 was found. See Fig. 1 d. I have verified this result on several occa- 



Pig. l. 



sions. I find that when rnicromeres are formed the reddish granules 

 of pigment found over the surface of the egg move away entirely from 

 the micromere pole. In the four cell stage, before the third 

 furrow has appeared, this movement of the granules is also seen to 

 be taking place in nearly all of the eggs and one can then see that the 

 movement takes place always at the non - nucleated ends of the four 

 cells. In some cases when a wide cross furrow is found, even then the 

 same change takes place involving a polar circle, including the large 

 clear area of two of the opposite cells, and a much smaller area of the 

 other pointed two. In the eggs of one female I distinctly saw this 

 movement of pigment take place at the two cell stage, although 

 in the other lots I have not found this to take place. I have never 

 observed anything of the sort in the unsegmented egg. The conclusion 

 follows that as early as the two cell stage the protoplasm 

 of the Arbacia egg is not isotropic but even at this 

 time the micromere field is foreshadowed. 

 The results may be considered under three headings: 



A. Study of egg-fragments. 



B. Effect of compression on the formation of rnicromeres. 



C. Effect of 2°/o additional salt to the sea-water. 



