328 DEVELOPMENT OF THE OVULE. 
a special cavity is formed at or near its apex (fig. 739, c), in 
which the embryo or rudimentary future plant is developed 
after fertilisation ; hence this cavity is called the embryo-sac. It 
is analogous to the megaspore (page 370) of cryptogamous plants. 
In rare cases, as in the Mistletoe, two or three embryo-sacs are 
formed. This sac is produced by the special development of 
one of the cells lying near the centre of the nucellus, which as it 
continues to increase in size presses upon the surrounding 
parenchymatous cells, and thus occasions their more or less com- 
plete absorption, This sac sometimes causes the almost entire 
absorption of the nucellus, and even projects beyond it, either 
through the opening in its coats afterwards to be described, 
called the micropyle (figs. 742, e, and 743, m), or through its 
sides in various directions, by which one or more saccate pro- 
cesses are formed. More usually, however, the tissue of the 
nucellus is not entirely absorbed, but a variable proportion is 
left surrounding the embryo- 
Fic. 737, 8a¢. _ The sac contains at first 
an abundance of watery cell- 
sap and protoplasm, in which, 
before fertilisation takes place, 
there are usually three rounded 
hy ess. i + or oval large nucleated cells 
Pasi; formed at its apex, which have 
ie Te been termed the germinal or 
t, f? embryonic vesicles (fig. 737, b). 
“>< Different views have been 
Aa entertained of the structure 
of these germinal vesicles. 
But they are now universally 
regarded as simply nucleated 
Fig. 736. Apex of the embryo-sac in the 
ovule of Polygonum divaricatum. 8s, 5S. masses of protoplasm, or, in 
Synergide. e. Oosphere-——Fig. 737. other words, primordial cells, 
The internal parts of the ovule a short 5S 
time before fertilisation. «a. Inner coat as will be afterwards fully 
of the ovule. s. Embryo-sac. b. Ger- explained when treating of the 
minal vesicles. ¢. One of the antipodal Reproduction of the Angio- 
py wee me spermia. Less frequently one, 
three, or more of these cells make their appearance. Sometimes 
the germinal vesicles are considerably elongated, being attached 
to the wall of the embryo-sac by the narrower end, and projecting 
by their free rounded extremity into the cavity of the sac. 
The two upper of these germinal vesicles (jig. 736) have been 
termed the synergide, s, s; and the third, which is placed 
somewhat laterally lower down, is the oosphere or ovwm-cell, e, 
which ultimately becomes the embryo, as will be explained 
hereafter (see Reproduction of the Phanerogamia). Besides 
these germinal vesicles, the embryo-sac usually contains, before 
fertilisation has been accomplished, two or more small nucleated 
cells which have been called antipodal cells (fig. 737, ¢), from 
