260 BRYOPHYTA 
that the event of reduction was deferred in the course of its descent. The 
cell-mass thus produced in Acca is at first homogeneous, as was probably 
the case definitively in certain of its ancestry. Differentiation comes 
later in the sporogonium of Zéccia, as it probably did also in the race: 
in place of every cell being equally liable to the tetrad-division, this is 
carried out only by those which lie internally: those forming the 
superficial wall are sterile, and form only somatic tissue. There is ample 
evidence of such sterilisation of fertile cells occurring elsewhere, both in 
plants related to Azccfa, and in other phyla (Chapter VII.), so that no 
& priori objection can be taken to its place in the theory: there is, 
however, no direct proof that this was actually the case. The remaining 
cells which lie centrally then all undergo the tetrad-division, which on 
the above theory was the primitive condition for all the cells of the 
sporophyte. 
Till recently it was thought that the fruit-body of Codeochaete supplied 
a prototype of an undifferentiated mass of cells, all fertile, such as this 
theory contemplates; but it has now been shown that in Coleochaete 
reduction occurs at the first segmentation of the zygote, and accordingly 
the old comparison is no longer permissible. There is, however, a growing 
body of evidence, from several distinct phyla of Thallophytes, that the 
event of chromosome-reduction consequent on sexuality may be deferred 
in the individual life: that a sterile, or vegetative phase of the nature 
of a sporophyte, varying in structure and in mode of origin, but similar 
in being partly somatic, partly fertile, may be thus intercalated between 
the two events. The Florideae, the Ascomycetous Fungi, and the Uredineae 
provide examples of such intercalation of a sporophytic phase: these point 
an analogy in this respect with the simplest Archegoniatae, though along 
phyletic lines almost certainly apart from the latter (Chapter V.). Thus 
the view now stated of the phyletic origin of the simple sporogonium of 
Ricia by continued segmentation of the zygote, and deferred tetrad-division, 
with sterilisation of the superficial cells, is in the main hypothetical, it 
is true; but it has a reasonable basis, partly on the facts of the individual 
development, partly on analogy. In the absence of still simpler sporo- 
phytes affording comparisons within the series of the Archegoniatae 
themselves, this analogy, together with the facts of the individual develop- 
ment in Azccia itself, make the view thus stated appear more probable 
than any alternative hitherto proposed. 
Ricca being the simplest type of sporogonium in the Archegoniatae, 
the basis of the antithetic theory has been fully stated, as applied to the 
facts of its development. The same theory may be extended from it to 
other forms also, in which the sporophyte, though more complicated, 
arises from the zygote by similar though more extended segmentation. 
Steps in advance are illustrated in other Marchantiales, which will now 
be described. 
The sporogonium of the Marchantiaceae, of which Fegatella (Conocephalus) 
