406 
concerned in the reproductive function, the stem segment. This 
is important from the fact that at the start the embryo is wholly 
dependent upon the prothallium for its nutriment. 
The posterior segment, being the larger, presents a much 
greater absorbent surface over the prothallial tissue, and by sub- 
sequent growth and cell division more quickly reaches the ex- 
tended surface characteristic of the foot in the early stage of the 
embryo. 
For some time in the later divisions of the embryo, the tissue 
derived from the posterior segment exceeds in bulk that derived 
from the anterior segment. This is recognized as bearing a defi- 
nite relation to the function of the different parts of the embryo. 
Why should not the impulse to this rational differentiation in the 
young tissues of the embryo exist in the egg and be manifested 
in the primary unequal division as seen in this example of Pteris 
serrulata? Another case was found in the embryo of Adiantum 
cuneatum, the anterior segment with its nucleus was smaller than 
that of the posterior segment. Goebel, Outlines of Classification 
and Special Morphology of Plants, p. 204, figures an embryo of 
Adiantum Capillus-veneris showing the anterior segment much 
smaller than the posterior segment. The nuclei are not shown, . 
and he gives no explanation of the unequal size of the two seg- 
ments. If the section were not perpendicular to the first division- 
wall, but oblique to it, the size of the cells might vary according 
to the focus plane of the microscope or the position of the sec- 
tion in the embryo. In the specimens of Preris serrulata and 
Adiantum cuneatum, referred to above, it was especially noted that 
the primary division wall was perpendicular to the section. This 
added to the fact that the nuclei were unequal in size is conclusive 
that the stem segment was smaller than the root segment. 
A few cases only would not be sufficient evidence to warrant 
the assumption that a general law of this nature exists, but it 
‘seems to me that it is sufficiently strong to warrant an enquiry as 
to its significance. It is desirable that an examination be made of 
anumber of embryos showing the primary division, with a view 
to see how far this unequal segmentation will hold good. The 
fact that in the Polypodiacez the foot is much more highly devel- oe 
