6 
W. H. Lang. 
different products of their development depending on the different 
conditions. It will be evident that the former view, it faced fairly, 
complicates the problem of the transmission of characters and that 
we should be on simpler lines if we could regard the specific cell as 
essentially similar throughout the life-history. 
We know that the fertilised egg and the spore do differ in one 
normally constant and recognisable respect, that the spore (and the 
generation derived from it) is haploid, while the fertilised egg (and 
the generation derived from it) is diploid. When this was first 
recognised the tendency was to see in this difference the cause of 
the difference in the two generations, that is to regard them in the 
first of the two ways distinguished above. Recent work has tended 
to make this position very doubtful. On the one hand, as we have 
seen, there are organisms with the cytological difference between 
the two generations, but, with little or no difference in the bodies 
of the sexual and spore-bearing organisms. On the other hand in 
certain deviations from the normal life-cycle in Ferns the distinct 
body-forms of the prothallus and fern-plant may appear, although 
the cells are throughout haploid or diploid. The question has been 
discussed by Farmer and Digby, who conclude that “ any cell the 
nucleus of which is provided with the necessary chromosomes, 
whether these are in single or duplicate number, is at least poten¬ 
tially endowed with the capacity of forming the starting point of 
the entire life-history, in so far as the grosser morphological 
characters are concerned.” What does appear to be necessarily 
associated with the cytological difference is the mode of reproduction, 
sexuality in the haploid, and spore-production in the diploid 
generation. In this respect the similar bodies of Dictyota differ 
just as do the unlike bodies of the two generations in the arche- 
goniate plants. 
We seem therefore entitled to assume that the haploid and 
diploid germ-cells have potentially the same morphogenetic 
properties, i.e., that under the same conditions they would give rise 
to similar bodies. This brings us to consider the second explanation 
of the difference of the two generations in Bryophyta and Pterido- 
phyta, viz., that it is due to the different conditions to which the 
germ-cell is exposed, in initiating the two generations. We have to 
enquire whether such a difference of conditions exists as may be 
assumed to account for the very different results. 
There is normally a great difference in the conditions under 
which the spore and the fertilised egg commence their development 
in all archegoniate plants. The spore develops free, in direct 
