74 Campbell . — On the Prothallium and Embryo of 
out. In the foot this is probably the case in both octants, as it 
was not possible to detect any regularity in the divisions after 
the very first one. In the other quadrants, however, the apical 
cell of one octant persists as the permanent growing point. 
The embryo retains, for a longer time than is usual its 
original nearly globular form, all the organs growing about 
equally, and some of the organs projecting beyond the others, 
as is common in most Pteridophytes, this being especially 
true of the cotyledon. 
The Cotyledon . — The cotyledon arises from the lower epi- 
basal quadrant of the embryo. Its direction of growth is 
determined in part by the first walls in the octants that 
compose it. The outer octant, usually at least, becomes the 
apical cell of the cotyledon. If the first segment of the apical 
cell is formed in contact with the octant wall, this throws the 
apical cell very much to one side of the median line of the 
embryo, and the axis of the leaf may in consequence be nearly 
at right angles to this. If, however, the first segment is nearly 
parallel with the basal wall, the apical cell will lie nearly in 
the centre, and the growth of the cotyledon corresponds, for 
a time at least, with the axis of the embryo. Even in the 
latter case, however, owing to considerable growth in the 
inner octant, the growing-point is pushed to one side of the 
median line, and the cotyledon seems to grow out laterally 
from the embryo. Whether seen from the side or from above, 
the apical cell is triangular, and is really a tetrahedron, from 
whose lateral faces successive segments are cut off (Figs. 88- 
94 L). Sometimes, especially in the earlier stages, the divi- 
sions in the segments present a good deal of regularity. Each 
segment is first divided into an inner and outer cell, and the 
latter then divides into two by a wall parallel with the side of 
the apical cell. Both inner and outer cells undergo further 
divisions, and it is not until a late stage that the primary 
tissues are clearly distinguishable. The young cotyledon has 
the form of a cone, but as growth upon the lower and inner 
surfaces is stronger than upon the upper, it soon begins to 
curve upward and outward. This is most plainly seen in 
