CELL SIZE AND NUCLEAR SIZE 81 
cell may indicate the position and direction of the coming cleay- 
age, and I maintained then and in subsequent papers (’97, 99, 
’02) that the position of the spindle and the size, position, and 
histological character of the daughter cells is the result of the 
structure of the protoplasm, and particularly of the polarity and 
symmetry of the cell. 
These conclusions have been confirmed by muchexperimental 
work on cell division, which I have completed but have not yet 
published. The position of the spindle and the plane of cleavage 
may be greatly changed, but the polarity and organization of the 
protoplasm remain unchanged, as I shall show in a future paper. 
Indeed it is very difficult to alter the polarity of any cell as Lillie 
(706, 709) has shown, and one reason for this is to be found in the 
fact, as I have discovered in Crepidula, that the cell axis, 1.e., 
the axis connecting nucleus and centrosome, can rarely be changed 
by artificial means. 
6G. Regulation in the cleavage process. Evidently connected 
with this persistent organization of the cell is the power of regu- 
lation which is shown in the cleavage of the egg as well as in the 
regeneration of adult parts. Whenever the size or constitution 
of blastomeres of Crepidula have been changed, or when cleavages 
have been suppressed, subsequent cleavages come back to the 
normal form so far as this is possible. The original disturbance 
can be righted only very gradually if at all, since neither yolk, 
cytoplasm nor nuclei can pass through cell membranes, and the 
only redistribution of substances possible is by means of new cell 
divisions. But in Crepidula the divisions following upon such 
a disturbance of the usual cleavage process are almost if not en- 
tirely normal. This is very evident in the divisions following upon 
disturbances of the first two cleavages. All of the yolk may be 
centrifuged into two of the macromeres and practically all of 
the cytoplasm into the other two, as in figs. 16, 19, 21, 23, et. al; 
two of the ‘macromeres’ may be very small and two very large, 
as in figs. 16, 21, 23 to 28; or one of these first two cleavages may 
be suppressed, as in figs. 22 and 26; but if such abnormal eggs 
are allowed to develop under normal conditions, the micromeres 
are formed in normal manner, as is shown in figs. 24 to 28 and 32 
THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 12, No. 1 
