466 SPERMATOPHYTES (SEED PLANTS) 
connection with double fertilization 1t should be noted that the 
endosperm nucleus contains the contents of three nuclei, since 
it is a product of a triple fusion, involving a sperm and the two 
polar nuclei. 
Embryo. — The first cells produced by the division of the fer- 
tilized egg form a filament which pushes down into the embryo 
sac. This filament is called the proembryo. The terminal cell 
of the proembryo develops the embryo, while the remainder of 
the filament remains as a stalk called swspensor. After the termi- 
Fia. 412. — Development of the embryo and endosperm in the Shepherd’s 
Purse. A, section through ovule with embryo and endosperm in early stage 
of development, showing the proembryo which consists of the suspensor (b) 
and the terminal three-celled embryo (a), and also showing the endosperm (c) 
as a chain of free nuclei around the wall of the embryo sac. B, the same as 
A, excepting that the proembryo and endosperm are more developed. C, 
section through a mature seed showing the seed coat (s), and the mature 
embryo with cotyledons at h, plumule at p, hypocotyl at e, and radicle at d. 
nal cell divides a number of times, the parts of the embryo begin 
to be differentiated. In Dicotyledons two lobes appear at the 
end farthest from the micropyle and these become the two coty- 
ledons characteristic of dicotyledonous Angiosperms. Between 
the cotyledons the plumule is formed, while the axis of the embryo 
below the cotyledons is differentiated into the hypocotyl, which 
is the main part of the axis, and the radicle at its lower end 
(Fig. 412). 
The embryos of monocotyledonous Angiosperms have a radicle, 
hypocotyl, plumule, but only one cotyledon. They also differ 
