A ffinities of the Palaeozoic Seeds of the Conostoma Group . 
21 
how far it is possible to reconcile them with the probable course of events 
in the history of ovular maturation. 
At an earlier stage of development than any represented by our pre- 
parations, the tip of the nucellus must have been occupied by a soft internal 
tissue which filled both lagenostome and plinth, the tissue in question being 
continuous from the one to the other through the narrow orifice by which 
these structures communicated 
(Text-fig. y, a). As the time of pol- 
lination drew near the tissue of the 
lagenostome doubtless underwent 
solution through the agency of ap- 
propriate enzymes spontaneously 
secreted. At this time the floor 
or septum which lay between the 
plinth and the megaspore cavity 
must have been arched right up so 
that it was separated from the base 
of the lagenostome by the thickness 
of the lens-shaped cushion of tissue 
which we find surviving in later 
stages. Peripherally this tissue 
must have extended considerably 
further than does its surviving rem- 
nant in any of our specimens, and 
in particular it must have occupied 
the curving sides or ‘ shoulders 5 
of the plinth. Text-fig. 7, A, may 
perhaps serve as a reconstruction 
of this early stage. 
As the solution of the core 
of the lagenostome advanced the 
enzyme-action would traverse the 
base and involve the tissues of 
the plinth (Text-fig. 7, b). 
To account for the next stage one of two assumptions has to be made : 
either the tapetal membrane (floor of plinth) tended to contract, or else — 
and we think this the more probable — the summit of the nucellus (including 
the lagenostome) continued to rise in consequence of a late extension 
localized in the plinth, whilst the tapetal septum remained stationary or at 
any rate lagged behind in its growth. Only in one of these two ways 
does it seem possible to explain the separation of the small central pad 
which rests on the tapetal septum from the lens suspended above (Text- 
fig. 7, c). 
Text-fig. 7. Diagrams of series of develop- 
mental stages (in part hypothetical) of plinth and 
lagenostome in a Conostoma , showing arrange- 
ments for entry of pollen into the plinth chamber. 
The dotted areas represent soft filling tissue. A, 
young stage ; B, cavity of lagenostome cleared of 
tissue and central patch of plinth tissue ready to 
separate as the plinth elongates (c) ; in D the 
prothallus has encroached on the plinth by means of 
a ‘ tent-pole ’ extension. Pollen present in c and d. 
