DOES NOT DETERMINE ORGANOGENY 179 
1870-1880 is untenable in face of present facts, still their reasoning was 
correct: and quite logically (provided the premises were sound) it was 
argued that if the segmentation at the apex of axis or root defines and 
dominates the later development of its tissues or appendages, then a similar 
importance, but enhanced by its earlier position in the individual life, should 
attach to the first segmentations of the zygote. Accordingly the study of 
segmentation was assiduously pursued back to the earliest stages of the 
embryo; and, as apparently confirming the position, the fact was disclosed 
that a high degree of constancy rules in the first fissions of the ovum of 
the Archegoniatae. Also it was found possible, with some degree of cer- 
tainty, to assign specific developmental functions to the earlier segments: 
thus the first or basal wall was seen to separate a part which habitually 
formed the shoot from a part which habitually formed the foot or root: 
further, the four quarters of the Fern-embryo were shown to correspond to 
the points of origin of stem, leaf, root, and foot: and as the Leptosporangiate 
Ferns were regarded about the time when this work was being done, as a 
fundamental type, the effort was made on the basis of the Fern-embryo, to 
construct what might be called a general embryology founded upon study 
of cell-cleavages. This was extended not only to the various types of 
Pteridophytes, but also, irrespective of the great systematic gulf which lies 
between these classes of Plants, to the Bryophytes. An example of the 
lengths to which this embryology based upon cell-cleavages was driven is 
found in the comparison of the embryo of a Fern and of a Moss, by 
Kienitz-Gerloff.1 He recognised the basal wall of a Fern-embryo as com- 
parable with that of a Moss: the epibasal half of the embryo in the latter 
divides into quadrants, of which one develops no further, while the other 
forms the whole of the upper part of the sporogonium. Since this quadrant 
corresponds in position, and in some degree in segmentation to that which 
forms the leaf of a Fern, it was suggested that there is a true homology 
between the sporogonial head of a Moss and the leaf of a Fern. Such 
comparisons die hard, and this one still figures in the morphological arena. 
A more reasonable position, and one which is likely to leave still more 
permanent effects on current embryology, was that of allocating certain organs 
of the embryo to certain octants resulting from the primary segmentation 
of the zygote. It is true that the cleavages are relatively constant in certain 
forms: and that the position in which the several parts originate may also 
show a high degree of constancy. The reference of such parts in origin to 
certain octants presupposes that there is some causal connection between 
the two. There are, however, good reasons for not conceding any such 
causal connection. The first is the fact, now demonstrated even in cases 
where the apical segmentation is regular, that the parts of the mature 
sporophyte are not referable in origin to definite segments. A second is 
that in many cases though the part in question may be referred in origin 
to a definite octant or octants, only a relatively small part of those octants 
1 Bot. Zeit. 1878, p. 55. 
