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EDWIN G. CONKLIN 
The first cleavage furrow then begins to appear. A ''cleavage 
head'' (Ziegler 1898) of clear protoplasm cuts into the egg over 
the upper hemisphere in the plane of the first cleavage (fig. 10). 
In this plane the clear protoplasm extends around to the vege- 
tative pole and the constriction then deepens all around the egg, 
and at the same time the line of demarcation between the clear 
and yellow hemispheres becomes indistinct. Finally the con- 
striction is completed and each daughter cell becomes an almost 
perfect sphere, in contact with its sister by onh^ a small area. 
The daughter nuclei appear as clear spots, the clear astral areas 
disappear and in their places the yellow sphere substance is ag- 
gregated, and the line of demarcation between the clear and yellow 
hemispheres of the egg becomes more distinct (fig. 11). Finally 
the portion of the yellow pole uncovered by the clear protoplasm 
becomes smaller, the daughter cells flatten together until they are 
nearly hemispherical in shape and the first cleavage is completed. 
At the close of the first cleavage a small lenticular space makes 
its appearance in the cleavage plane between the daughter cells, 
apparently from the region of the mid body (''Zwischenkorper"), 
and it increases in size until it extends nearly to the periphery of 
the egg (figs. 12,13). This is the ''intermittent cleavage cavity'' 
of earlier investigators. 
The second cleavage then begins; the mitotic figure appears as 
a clear elliptical area surrounded by the yellow sphere substance 
(fig. 13) ; the lenticular cleavage cavity disappears and the clear 
protoplasm, which covers the entire upper hemisphere, may be 
seen to dip down into the first cleavage plane in the place of this 
cavity (figs. 13, 14). The second cleavage is completed in the 
same manner as the first. 
From the 4-cell stage onward the substance of the yellow hemi- 
sphere is gradually encroached upon by the clear protoplasm until 
it is limited to a small area around the vegetal pole (fig. 15). The 
first, second and third sets of micromeres (ectomeres) are formed 
and they contain little, if any, of this yellow substance, all of 
which is left in the macromeres (figs. 16-21). In all subsequent 
divisions of the macromeres to form the mesoderm and endoderm 
cells of the fourth quartet, this yellow substance is distributed 
