632 
T. H. MORGAN 
Still another observation points in the same direction. In 
some eggs — the frog's for instance — the first division may slowly 
pass through the yolk hemisphere, while the preliminary steps 
for the next division are going on in the region of the nucleus. 
I have pointed out that the spindle for the second division may 
actually develop before the first division is completed. Evidently 
protoplasmic division once begun may continue when the nucleus 
has passed or is passing into its next phase. Such facts are difficult 
to interpret on the assumption that the nucleus is the active cen- 
ter of the change. But in these eggs the peripheral astral sys- 
tem may remain and possibly continue its development into 
remote parts after its original center has divided into two. The 
presence of this peripheral system may suffice to complete devel- 
opment once begun. Yatsu's experiments with egg fragments 
point to the same conclusion. Other evidence might also be ad- 
duced to emphasize the importance of the astral system in divi- 
sion but this will suffice to show its significance. How then shall 
we interpret the action of the aster? Two suggestions offer them- 
selves for consideration: First, the system of rays stiff er than the 
rest of the cytoplasm may represent a skeleton or supporting 
frame work for the mechanism involved in the division. Such an 
interpretation would still leave the essential features of the pro- 
cess to be explained. Second, the formation of the rays may cause 
a change in the surface tension of the cell leading to a constriction 
of the mass about the astral centers. If, as I have suggested, the 
formation of the rays represents a molecular change in the ground 
substance, such a change might involve an alteration of the inte- 
rior of the cell in contrast with its surface, and from a physical 
point of view such changes taking place around two centers might 
be made to account for the constriction of the cell. Whether these 
ideas have any value as working hypotheses the future must show. 
The observations here recorded suggest at least the possibility 
of analysis along these lines. 
