82 THE BRITISH FERNS. 
diameter, so as to give the whole a cone-shaped outline in the side 
view. 
It will thus be seen that, according to Professor Henfrey’s 
explanation, the antheridia consist of a large cell, having thick 
walls consisting of a double membrane, and containing free 
cellules, which are the spermatozoids coiled up within a membrane, 
from which he has found them in some instances apparently 
unable fully to extricate themselves. We learn, however, from 
Dr. Lawson, that his observations lead him to conclude that it 
is the cell-wall of the free cellules, which itself unrolls to form the 
spiral filaments, and that they are not invested by any external 
membrane. 
At a later period of the growth of the prothallus, other larger 
and more complex cellular bodies appear, which are analogous to the 
ovules or nascent seeds of flowering plants. These bodies, called 
‚pistillidia, or archegonia, are much fewer in number than the anthe- 
ridia. They are formed near the centre of the prothallus, and at 
first consist of a single cell of the (inferior) surface, destined to 
become an embryo-sac. This, by a process of cell division, becomes 
enlarged, so as when full grown to acquire the appearance of 
papille, of a bluntly conico-cylindrical form, and composed of 
about four tiers of four quadrant-shaped cells. In the embryo- 
sac at the base, a central globular bedy, which forms the germinal 
vesicle, is at length produced, and a clavate cavity or canal 
extends from this between the convergent inner angles of the 
four series of cells, up to the apex of the papilla. The ger- 
minal vesicle becomes fertilised through the agency of one 
of the spermatozoids, or spiral filaments, which enters by the 
canal, and an embryo plant possessing a terminal bud is gradually 
developed. 
With regard to the fertilisation, Professor Henfrey is of opinion 
that the operation is effected by the contact of one or more of the 

