TETRAD AND EMBRYO-sac MITOSES 85 
On similar grounds the failure of the megaspores to form cell 
walls might be urged against the interpretation of these cells as 
such, but here again this character has dropped out without, it 
seems to me, invalidating the view that ‘the cells in question are 
spores which are thereby removed one step further from their more 
primitive condition in which a cell wall was eminently necessary. 
To be sure we are not rid of the conclusion that, in those plants 
in which the embryo-sac arises directly from the mother-cell the 
embryö-sac is constructed, so to speak, of four spores ; or that, 
when the mother-cell suffers one division giving rise to two 
daughter-cells of which one only is functional, the embryo-sac is 
constructed of two spores. But after all, spores, in the sense here 
meant, are equivalent vegetative cells of a somewhat special sort, 
but with no necessarily separate existence, teleologically speaking, 
except in those plants in which the spores are the functional equiv- 
alent of the seeds in the higher plants. 
The conclusion is therefore drawn that the cells under discus- 
sion are distinguishable by their origin, that they are not cut out 
of the cycle of development and that their morphological. persist- 
ence is connected with the function of the mitoses; while on the 
other hand, the spore “as such * * * is wanting" * because the 
particular features of specialization which characterize spores have 
been lost in connection with the loss of the more primitive spore 
function. 
The following question is very naturally raised at this point, 
namely, at what point in the life cycle does the gametophyte com- 
mence its existence. There can of course be no categorical, or 
at least no final answer given under our present understanding, 
But we may state as fairly in accord with the facts, that the mor- 
phological identity of the gametophyte has been reduced along with 
the change in the function of the spores, and the two periods of 
development have been merged into each other, to different degrees 
in different plants. In the most extreme cases, as Zilum, the 
gametophyte appears to commence histologically with the embryo- 
* Atkinson, Z. c. The italics are his. 
t Coulter and Chamberlain in their recently published volume ** Morphology of the 
Spermatophytes ’’ (New York, 1901), take a tentative position. In their ene 
**close the history of the sporophyte with the appearance of the spore mother-cell, 
Since it seemed to them “* to be the best defined line of demarkation ” (page iv). 
