IN THE BRYOPHYTA 203 
of the products. Such a condition is seen in the spherical sporogonium 
of £&iccta, which has habitually been held to be primitive in its simple 
characters of structure and form (compare Fig. 18). Here there is no 
polarity: no distinction of apex and base. This character it shares with 
the earlier stages of some other embryos of Archegoniatae, which enlarge 
at first as a simple sphere. But a distinction of apex and base soon 
makes its appearance in all the more complex. forms, with or without a 
localised apical growth. The two great series of Archegoniatae differ 
widely in the symmetry of their further development. The Bryophyta, with 
very few exceptions, which will require special consideration, show polarity, 
but retain their radial symmetry. Not a few of the Pteridophyta also 
retain their radial symmetry, but under modifications which necessarily 
follow as a consequence of their leafy habit: others, however, depart 
broadly from it, some at an early period of their individual life, others 
at later periods. 
The general view which is implied in the preceding paragraph is that 
the radial type of symmetry is the prior condition for the sporophyte at 
large. This opinion is not based merely on the fact that the ovum from 
which all sporophytes spring is spherical. Much stronger grounds are to 
be found, first, in the high degree of constancy of the radial type of con- 
struction in the sporogonia of Bryophytes: while it is also frequent in 
the Pteridophytes and Seed-Plants, especially in their strobili and flowers. 
Secondly, in the fact that it is possible in many cases to refer the dorsi- 
ventral symmetry, where it exists, to the unequal incidence of external 
conditions, and to see by experiment how such conditions may bring 
about some dorsiventral modification of a structure which is in the first 
instance radial. Examples of this may be quoted occasionally from the 
Bryophytes, and frequently from the Pteridophytes, and from the vegetative 
shoots and flowers of Phanerogams. There is thus not only a compara- 
tive, but also an experimental basis for the opinion that the radial 
symmetry is the primitive, and the dorsiventral the derivative condition in 
the sporophyte. 
Few facts relating to any large group of organisms are more 
impressive than the constancy of the radial symmetry throughout the 
sporogonia of Bryophytes. That body, originally spherical, becomes more 
or less spindle-shaped in its later development, with or without a localised 
apical growth. Zones higher or lower on the spindle-shaped body may 
undergo more strong development than the rest, especially towards the 
distal end, which is to be the fertile capsule. This is commonly seen, 
both in Liverworts and in Mosses, but the Splachnaceae stand out as 
extreme examples, and in Splachnum luteum the apophysis immediately 
below the capsule is expanded into a wide disc (Fig. 102). Nevertheless, 
here also the development is uniform all round in any transverse zone, 
and accordingly the radial construction is accurately maintained. The 
constancy thus usual for the sporogonium in itself directs attention to 
