56 Farmer. — On Isoetes lacustris 3 L. 
formation of the archegonia, and so similar are the earlier 
formed cells of the outer layer all through this upper portion 
of the prothallium that it is impossible, until quite late, to pre- 
dict which are destined to give rise to archegonia ; all are 
apparently, and to an equal extent, potential reproductive- 
organ-forming cells. The archegonia are formed by the peri- 
clinal division of an outer cell into two daughter- cells, the outer 
of which forms the four (sometimes three) stories of neck-cells, 
whilst from the inner one are cut off successively the neck-canal- 
and ventral canal cells, leaving the oosphere at the base. While 
these divisions are taking place the surrounding cells divide 
into a small-celled tissue in which the archegonium is buried, 
only its outermost neck-cells being free. The venter is thus 
formed, as in the highest Vascular Cryptogams and Gymno- 
sperms, not from the mother-cell of the archegonium, but from 
the cells of the surrounding tissues. The neck-cell, when first 
cut off, divides into four cells arranged cross-wise, and then 
each of the four is divided into two cells transversely, and 
each of these again divides, once more forming the four stories 
of which the mature neck is commonly composed (PI. V. Figs. 
20, 21). The neck-canal-cell grows in between the neck-cells, 
thrusting them apart, and establishing, eventually, a connec- 
tion between the exterior and the inner part of the archegonium. 
The neck-cell and the ventral canal-cell all finally become mu- 
cilaginous as in other Cryptogams. 
Whilst these changes are rapidly proceeding in the upper part 
of the spore, the larger basal portion also undergoes changes. 
Its protoplasm is never so abundant as that of the upper 
cell-tissue, but the reserve material stored up in it is far more 
plentiful. It forms a cellular mass, but comparatively slowly, 
beginning at the periphery, and fortunate preparations show 
the large peripheral cells enclosing a central mass of protoplasm, 
as yet undivided, but in which are lying several free nuclei 
(Fig. 19). The whole appearance recalls the free cell-formation 
taking place in an embryo-sac during the formation of endo- 
sperm, though there is nothing more than analogy between the 
final products in the two cases. The lower portion of the spore 
