178 SEGMENTATION 
generalisation came to be widely accepted that the delimitation of the 
embryonic tissues by Hanstein should coincide exactly with that of the 
mature tissues by Van Tieghem, and that this is generally applicable to 
stems as well as roots of Vascular Plants. 
But this whole subject has recently been submitted to a detailed 
revision by Schoute,! and it has been shown that the generalisation of 
Hanstein—that the three formative regions exist at the apex—was based on 
a very narrow area of observation. As a matter of fact, there is no separate 
origin of them in Pteridophytes, for they all spring from the initial cell or 
cells. In the Phanerogams such separate origin is best seen in roots, but 
even there it is not constant at the extreme tip.. In stems it is only seen 
clearly in some few cases; in most stems hardly at all. The dermatogen 
is the most frequently and the most clearly defined of the three: the 
distinction of periblem and plerome in stems is only rarely carried out. 
Neither is the partitioning of the formative regions in the embryo clearly 
marked at an early stage in vascular plants at large: though there is some 
approach to it in some Dicotyledons, the Monocotyledons and Gymno- 
sperms show little regularity, and it is almost entirely wanting in some 
plants. Thus it cannot be said that the details of development of the 
embryo in any way strengthen the position as regards the formative regions 
of Hanstein. The general conclusion seems a justifiable one, that no 
great morphological importance is to be attached to the formative regions 
of Hanstein since they are so commonly of inconstant occurrence. 
An examination of the further question whether the dermatogen really 
produces epidermis, the periblem cortex, and the plerome the central 
cylinder, also shows inconstant results. In those roots in which periblem 
and plerome are clearly defined at the apex the cortex does originate 
from the periblem and the central cylinder from the plerome, though this 
does not hold exactly for ail roots. But in stems the correspondence of 
Van Tieghem’s primary tissue-systems with the formative regions does not 
hold: it is not even carried out exactly in the single regular example 
which has been described, viz. Aippuris: for here, according to Schoute, 
the endodermis and part of the cortex may be derived from the plerome. 
It is thus seen that the case is similar to that already discussed of the 
relation of apical segmentation to the origin of the appendages: in certain 
few examples the early segmentation may coincide with the definite mature 
condition, but in the great majority no such regular relation exists in 
either case. Where it does exist it may be held to be casual rather than 
causal, and will bear no constant phylogenetic significance. 
The reasonable degree of success which seemed at first to attend these 
efforts to correlate with early segmentation at the apex not only mature 
external form, but also internal structure, led to a peculiar development 
in the study of the primary origin of the embryo from the egg. However 
clearly we may now see that the position assumed by the investigators of 
1 Die Stelir Theorie, Groningen, 1902. 
