239 
Life-History of Isoetes . 
part remains undivided. The whole process recalls most 
vividly the endosperm-formation in most angiosperms. Owing 
to the dense contents, and the extremely thin cell-walls, it is 
not easy to determine exactly when the whole cavity of the 
spore becomes filled with the cellular tissue. On account of 
the preponderance of the free nuclei in the upper part of the 
spore, and their consequent proximity to each other, the cells 
of the prothallium in this region are smaller than those in the 
lower and central part of the prothallium (see Figs. 10-12). 
The divisions in these upper cells, too, are more frequent, and 
sometimes the transition from the upper small-celled tissue 
to the lower large-celled is quite abrupt, and all the more 
noticeable as the upper cells are comparatively free from the 
coarse granular contents of the lower cells. After the first 
cell-walls are complete, the subsequent nuclear division is 
accompanied regularly by the formation of division-walls be- 
tween the daughter-nuclei. 
The process here described it will be seen is very similar to 
that found in the usual endosperm-formation of the sperma- 
phytes, and differs very much from Hofmeister’s account 1 , 
according to which free cells are first formed that afterwards 
coalesce to form the prothallium. Only in a very few instances, 
and these mostly later stages, was any appearance noted that 
had the appearance of a splitting of the protoplasm, as Farmer 2 
describes, and this was readily shown to be due to shrinkage 
caused by the action of reagents, and with a little care it was 
evident that in all such cases cell-walls were present, and that 
the apparent cracks simply marked their position (Fig. 40). 
The first archegonium is very early evident, generally before 
the cell-division is completed in the lower part of the spore. 
The mother-cell (Fig. 10, a) occupies the apex, and is early re- 
cognisable on account of its larger size and denser contents. It 
is simply one of the first-formed cells that ceases to divide after 
it is complete ; and as the neighbouring cells divide rapidly, 
the contrast in size between it and those adjoining becomes 
very marked. Whether seen from above or in profile, it usually 
1 Loc. cit.,.p. 339. 2 Proceedings of the Royal Society, vol. 45, p. 307, 18S9. 
