54i 
Dull. — Pityostrobics macrocephalus , Z. and H. 
mass of substance, which makes it practically impossible to determine the 
nature of the tissue. The evidence is in favour of the view that the basal 
part, at least, is hollow. In the ovule seen in Fig. io, PL XV, the opaque 
body appears to extend right into the tissue of the prothallus, but even here 
the bounding layer of the megaspore is found to be continuous round 
the intrusion. In some cases where the longitudinal section of the ovule 
is rather out of the median, the beak is not represented at all, but its 
position is indicated by a gap in the nucellar tissue which stretches 
across the apical region ; the edges of the gap being somewhat upturned. 
This appearance (see M. y Fig. 3, PI. XV) lends support to the view that the 
base of the beak was originally hollow. In one of the ovules from a small 
tangential longitudinal section there occurs a somewhat similar, but much 
narrower gap in the upper part of the nucellus, which is peculiar in being 
continued below through the megaspore-membrane as a sharply curved 
channel in the tissue of the prothallus. At the margins of the channel 
can be seen elongated cells somewhat like those of the nucellus, but larger 
and with more resemblance to the cells of the endosperm tissue, which, 
as a matter of fact, can only be made out in this particular slide. It 
is possible that the cavity has been formed by the invasion of the pro- 
thallus by a pollen-tube, although no trace of the pollen-grain can be 
found. It must, however, be remembered that the section is very oblique. 
(iv) The Female Gametophyte and Embryo . 
All the ovules seen in the larger sections contain prothalllal tissue at 
about the same stage of development, but the actual state of preservation 
is somewhat different in different sections. An unusual condition is to 
be found in an ovule from the base of the cone of P, ovatns (Fig. 10, 
PI. XV), the centre of the ovule being occupied by a small contracted 
megaspore with a much crumpled outline. In the majority of cases the 
prothallus is fairly large, though its membrane has often been somewhat 
contracted. Very often, especially in the Sedgwick Museum cone, there 
seems to have been little or no shrinkage, the outline being well rounded 
and closely surrounded by the nucellus (e. g. Fig. 9, PL XV). 
The megaspore membrane consists of a comparatively thin brownish 
layer. Traces of prothallial tissue can nearly always be found within the 
megaspore ; as a rule the individual cells are not distinguishable, though 
there is a close network of faint yellow lines and granules which is clear 
enough evidence of the nature of the tissue. The individual cell-walls 
could only be distinguished in one of the tangential sections where the ovule 
had been cut across transversely, and where at one side the prothallial 
tissue had contracted from the megaspore wall. In this region the outer 
part of the prothallus is seen to be composed of very thin-walled, more or 
less isodiametric, polygonal cells. 
