45 6 Oliver . — The Ovules of the older Gymno sperms. 
excavated in the apex of the nucellus through a mucilaginous 
breakdown of the tissue there ; and should the watery muci- 
lage tend to concentrate through desiccation, fresh supplies 
of water would be drawn up from the tracheal system. Ulti- 
mately there is reason for supposing that the way to the 
archegonia (which lie much as in Cycas ) was cleared for 
the passage of the spermatozoids by a further mucilaginous 
breakdown in that portion of the tracheal sheath which 
overlaid the summit of the macrospore ; but in the tracheides 
outside the area of the pollen-chamber no such change is 
indicated. Whether it may not have happened in allied 
seeds that the nucellar tracheides stopped short from the 
first at the margin of the pollen-chamber, so that the necessity 
for their solution prior to fertilization did not arise, is a ques- 
tion difficult to answer. For it must be borne in mind that it 
would be difficult to discriminate between such a case, and 
one in which the tracheides had been locally absorbed. That 
is, of course, unless the state of preservation were remarkably 
good and the appropriate developmental stages forthcoming. 
As regards the number of pollen- grains usually present in 
a pollen-chamber, it is impossible to speak other than broadly 
in consequence of the fact that even when a series of sections 
is obtained there is a considerable loss as a result of cutting 
and grinding. In the case of Stephanospermum , from twelve 
to twenty does not seem too generous an estimate ; and if 
each of the twenty cells or so of which each pollen-grain con- 
sists be regarded as producing a single spermatozoid, that 
would allow from 240 to 400 of the latter. The distance to 
be traversed in the passage from the pollen-grain to the 
archegonium varies in this seed from .5 to *85 mm. 
It will have been gathered from the foregoing that whilst 
the problem of water-supply in relation to free-swimming 
spermatozoids stood on a satisfactory footing, there still re- 
mained room for advance in the direction of greater precision 
in the mechanism as a whole. We still appear to have the 
promiscuous liberation of motile spermatozoids reminiscent 
of a heterosporous Pteridophyte. 
