1900] LIFE HISTORY OF SILPHIUM 107 
The amount of protoplasm in the developing sac is compara- 
tively small, frequently resulting in the presence of very large 
vacuoles. It will be seen also that the enlargement of the sac, the 
division of its nuclei, and the destruction of the surrounding cells, 
do not proceed at the same rate. Thus, in Jig. 34 the two nuclei 
have not left their position in the central strand of protoplasm 
in which they were formed, while there is no trace of the poten- 
tial megaspores, and the nucellus is stretched almost to the point 
of breaking; in fig. 35, on the other hand, the nuclei are already 
widely separated, whereas the sac itself is smaller than in SJE. 34; 
the remains of the potential megaspores are still recognizable, 
and the nucellus itself is in a fairly good condition. 
The structure of the mature embryo sac may perhaps best be 
described by taking that of S. zntegrifolium as a type, and com- 
paring the other species with it. As shown in fig. 38, the sac is 
several times longer than was the nucellus. It attains its great- 
est diameter at about one third of the distance from its micropy- 
lar end, and from this point it tapers quite uniformly toward the 
antipodal region. The egg apparatus occupies about the outer 
quarter of the sac. The synergids (sy) always lie, the one 
anterior to the other, in one side of the sac, the opposite side 
being occupied by the oosphere (0), which is attached well up 
toward the apex of the sac. It is quite common for each syner- 
gid to have a large vacuole in each end, with the nucleus lying 
between them. In jig. 38 the egg lies above the synergids, 
almost covering one of them, whose projecting margin is shaded 
deeply for the sake of distinctness. The reconstruction of the 
drawing from adjacent sections also made it necessary to repre- 
Sent the vacuoles and nuclei of the synergids as if showing 
through the protoplasm of the egg. 
The oosphere shows a large vacuole in its broad upper por- 
tion. Its nucleus is larger than those of the synergids, and con- 
tains a very fine chromatin network in resting condition, and a 
large and often vesicular nucleolus. After the fusion of the 
polar nuclei, which occurs near the middle of the sac (fig. 37), 
the primary endosperm nucleus approaches the egg apparatus, 
