82 EDWIN G. CONKLIN 
to 37. Whatever the content of the different macromeres may 
be, whether purely protoplasmic or entirely yolk the micromeres 
are always protoplasmic, even though division must be delayed 
until the cytoplasm which goes into the micromeres can be formed 
from yolk (figs. 24, 31, 35); whatever the size of the macromeres, 
the micromeres formed from them are approximately normal in 
size, even though yolk-rich cells must give up most of their cyto- 
plasm (figs. 24, 31, 35), or protoplasmic micromeres must divide 
equally (figs. 25, 28), in order to give rise to micromeres of the 
usual size. 
Such regulations of cleavage are probably caused, in the case 
of Crepidula, by the persistent polarity of each cell, which in 
turn leads to the localization of the spindle in a definite axis, 
with its pole at a definite distance from the surface of the cell. 
In what manner the polarity of the cell may cause the localiza- 
tion of the spindle is clearly shown in the cleavage of Crepidula. 
In former publications (99, ’02) I have called attention to the 
fact that definite movements of cell substance take place in divid- 
ing cells, and that these movements serve to orient the spindles; 
these movements are always related to the polarity of the cell 
and to that of the entire egg. Furthermore, I have elsewhere 
(02) called attention to the fact that the cell membrane is weakest 
opposite the poles of the spindle. I was formerly of the opinion 
that this was due to some influence of the spindle on the cell 
membrane, but a further study shows that these weak places in 
the cell membrane are present before the spindle forms and can 
not therefore be caused by the spindle. In the egg shown in fig. 
29 the places of reduced tension on the cell membrane are indicated 
by the lobes of yolk attached to the cells, and a line drawn through 
the centrosome, nucleus and lobe indicates the precise position 
which the spindle will take at the next cleavage. The axes of the 
third cleavage spindles are here marked out long before the spin- 
dles are formed; the weak spot in the cell membrane is not caused 
‘by the position of the spindle, but the latter is the result of the 
former. Experiments on eggs in the 2-cell, 4-cell and 8-cell stages 
of cleavage show that the positions of the points where the mem- 
brane is weakest, change in each cell generation and that they 
