a a ee ee ee ee ee ee eee 
1897 ] THE LIFE HISTORY OF SAGITTARIA VARIABILIS 265 
a certain fixed course of development agrees neither with our 
present ideas of development nor with observed facts. 
Taking up again the general course of embryonic devel- 
opment, we have next the differentiation of the apical area 
of the stem, which begins in a hypodermal cell of tier 6 
(fig. 67). Another transverse division of the cell f (fig. 67) 
occurs, while at the same time the cotyledon also undergoes 
further development (jigs. 67, 68, 69). It will be noticed, 
therefore, that the cotyledon is the earliest region to be devel- 
oped, and that the apex of the stem follows. The development 
of the apical region of the stem is continued by transverse 
divisions of the neighboring dermatogen cells of tier 6 (fig. 69), 
and later the remaining cells of this tier also divide by trans- 
verse walls (fig. 70). At this stage the vesicular suspensor 
cell appears to be in its most active condition, but from this 
time on it begins to disorganize. At this time, and for some 
time later, the entire embryo is meristematic, and division may 
take place in any part. In the meantime, after considerable 
growth, the cell g (fig. 70) divides by a transverse wall, forming 
another tier #, the lower cell dividing again longitudinally into 
four cells (fig. 77). Whether tiers e and f (fig. 77) arise from 
tier e (fig. 70) I could not determine, although from the differ- 
ence in size of the cells of the two tiers it seems probable that 
they do not. The embryo now begins to elongate, showing a 
deep depression on the side where the stem apex is situated, 
and there is a farther development of cells between the 
embryo and the vesicular cell (fig. 72). At this stage the 
embryo sac is of almost mature proportions, and the embryo as it 
8tows downward bends around the curve of the sac, very likely 
because of the mechanical resistance offered by the walls within 
which it is confined, and thus acquires its hooked form ( jig: 73-) 
DIFFERENTIATION OF DERMATOGEN, PERIBLEM, AND PLEROME. 
The development of dermatogen begins at the apex of the 
Ro cotyledon, and as the embryo develops the dermatogen extends ee 
ee farther and farther toward the point where the apex of the Foot a 
