EMBRYO*SAC OF GYMNADENIA GONOPSEA, 
9 
In fig. 28 each of the four cells has doubled itself, and the 
power of division in the pro-embryo seems now to become 
exhausted, vacuoles begin to form, and the cells to elongate 
as their nuclei go to the walls, and the vacuoles collect into 
a sap-cavity (cf. figs. 28 — 30). Only in a few cases have I 
seen more than eight cells formed by the pro-embryo; in the 
specimen figured at fig. 30 are ten. As the cells elongate, 
since the solid embryo soon completely fills the embryo-sac, 
the apex of the pro-embryo becomes gradually pushed 
through the top of the sac, and the loose tissue of the micro- 
pyle allows it to escape into the cavity of the ovary (figs. 
29, 30); its period of growth is now about completed, and, 
as the last divisions are made in the embryo, the pro-embryo 
turns brown and shrivels up, persisting as a mere ragged 
appendage in the ripe seed. 
To return to the embryo proper consisting of two cells (fig. 
20). It becomes broader, and the nucleus of one cell divides 
and a new wall cuts it into two equal cells arranged laterally. 
The plane of this new division is always perpendicular to that 
of the first (horizontal) wall, and passes through the longer 
axis of the whole embryo ; it may appear first in the upper or 
in the lower cell, but usually the latter (fig. 21, 22). The other 
cell divides at the same time or very soon after by a wall, 
also passing through the long axis, and also perpendicular 
to the horizontal wall, but it is, in the majority of cases, if 
not always, also at right angles to the other perpendicular 
wall. Thus, in fig. 23 the first wall (horizontal) is cut at 
right angles by the second (perpendicular), which lies in the 
l)lane of the paper and in the upper cell ; the third (perpen- 
dicular) will cut both of these at right angles or nearly so, 
as shown by the dividing nucleus in the lower cell. Similar 
relations are shown in figs. 24, 25. Each of the four cells 
thus formed rapidly becomes again divided by a wall perpen- 
dicular to all those which it cuts, and passing through the 
long axis of the embryo, and in this manner the embryo 
comes to consist of a nearly globular body cut into eight 
octants, in each of which is a large round nucleus (fig. 26). 
The next division walls are again horizontal, and may 
appear first in the upper (fig. 29) or in the lower cells 
(figs. 27, 28); thus, the embryo becomes cut into twelve, and 
then sixteen cells, by walls in planes symmetrically related. 
A series of walls very soon mark out a central from an epi- 
dermal system; these (fig. 30) appear at about the same 
lime in all the cells except the tour which abut upon the pro- 
embryo, and lie in planes parallel to the outer wall of each. 
They may well be called tangential, and mark the first in- 
