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. Zdt. 1878, p. 55. 



