ITS BIOLOGICAL ASPECT 181 
till it is established in the soil. And as the prothallus differs in form and 
position, in size and in duration of life, so the germ itself may differ in the 
place and time of origin of its parts, as well as in their form and structure. 
A. few illustrations will show how this point of view gradually asserted itself. 
In 1882 a comparative revision of those parts which serve as haustoria 
in various embryos led me to the conclusion that they are not to be regarded 
as clearly defined morphological members, but rather as swellings of the 
hypocotyl, which arise only where they are required for the first processes 
of development and nutrition of the young embryo.! Some years later 
Treub introduced his theory of the “ Protocorm”:? theoretical considera- 
tions of the biological condition of the young embryo had led him to 
conceive of an organ preceding in descent the leafy shoot, such as is now 
seen in Vascular Plants; and this he recognised as actually present in the 
embryonic tubercle of certain Lycopods: a preliminary stage, in fact, which 
is to them as the protonema is to a Moss. Whatever view we may now 
hold of the protocorm, this theory takes its place as a further step towards 
a biological rather than a purely formal study of embryology. At the hands 
of various other writers such views have been further developed, especially 
in relation to the better knowledge recently acquired of the embryology 
of the Lycopods and Ophioglossaceae: and it was thus open to Goebel to 
formulate the position, as he does in his Organography.2 Having shown 
that external forces do not come into consideration in the arrangement in 
space of the parts of the embryo,* he points out that we need only consider 
internal factors, and say generally that root, shoot, and haustorium are laid 
down in the positions that are most beneficial for their function. This 
is in fact an extension to the whole embryo of the conclusion which I 
had applied in 1882 to the foot only. Comparative embryology of the 
sporophyte would thus become essentially a study of the circumstances 
and conditions which influenced the embryo during its evolution, and of 
the way in which the germ is formed to meet them. 
' But it may be enquired whether the germ itself does not still show 
beneath these adaptive modifications, some characters of a central type? 
Is all trace of the early evolutionary history eliminated by the subsequent 
modifications? There is at least one leading feature which remains traceable 
with some degree of constancy throughout the series of known embryos 
of the Pteridophyta: it is found in the relation of the parts to that initial 
polarity which is established at a very early stage in them all. This may 
often remain obscured owing to the precocious development of certain 
parts, in response to biological requirements; but nevertheless, it will be 
shown as the several embryos are described, that the apex of the axis has 
constantly a position in close relation to the intercrossing of the octant- 
1 Quart. Journ. Micr. Sct., xxii., p. 277. 
2 Buitenzorg Annals, viii., p. 1. The Theory of the Protocorm will be discussed at 
length below, in relation to the embryogeny of the Lycopods. 
® Organography, ii., p. 246. 4 Lbid., i., Pp. 219, 
