LIFE HISTORY OF AN ANGIOSPERM 
55 
the exit door for the hypocotyl of the embryo after the fertilized 
and ripened ovule becomes a seed. Within the nucellus, if the sec¬ 
tions examined have been properly fixed, will be found a megaspore 
or embryo sac. 
Development of the Female Gametophyte Through the Matura¬ 
tion of the Embryo Sac.—In its immature condition the embryo 
sac (megaspore) contains a mass of protoplasm surrounding a nucleus. 
This nucleus undergoes three divisions forming as a result eight 
nuclei which ultimately arrange themselves within the protoplasm 
of the embryo sac as follows: three of them occupy a position at the 
apex, the lower nucleus of the group being that of the egg or ovum, the 
other two nuclei being the synergids or assisting nuclei; at the oppo¬ 
site end of the sac three nuclei known as antipodals take their posi¬ 
tion; the two remaining nuclei called polar nuclei take up a position 
near the center of the embryo sac. In this condition the contents 
of the embryo sac constitutes the female gametophyte. See Fig. 
28 (1-8). 
MATURATION OF THE POLLEN GRAIN AND FORMATION OF THE 
MALE GAMETOPHYTE 
The pollen grains (microspores), within the anther‘sacs, all arise 
from a number of tetrads (groups of four) which are formed by the 
division and redivision of pollen mother-cells preceeding them. Each 
pollen grain, after the tetrads have separated into their components, 
consists of an outer firm wall or exosporium, an inner wall or endo- 
sporium, within which will be found .the region called the fovilla, 
which is nothing other than a mass of protoplasm containing a 
nucleus. Before the pollen is shed from the anther its protoplasmic 
contents undergo a series of changes leading up to the development 
cf the male gametophyte. The nucleus and protoplasm enveloping 
it divides to form two cells, one a generative-cell containing a genera¬ 
tive nucleus, the other a tube-cell containing a tube nucleus. The 
generative nucleus then divides to form two sperm nuclei and the 
partition wall between the two cells disappears. In this condition 
the protoplasmic contents of the pollen grain constitute the male 
gametophyte. 
