Alternation of Generations based on Ontogeny. 5 
siphonia) we appear to have an alternation of generations strictly 
comparable to those in archegoniate plants. Certain individuals 
bear the sexual organs, and from the fertilised egg, or its equivalent, 
a distinct generation is produced. In the form and organisation of 
the body the plant of this generation is almost exactly like the 
sexual plant, but its cells have nuclei with twice the number of 
chromosomes and the reproductive organs are different. The re¬ 
productive cells of this second generation arise by the division into 
four of a cell; in this division the number of chromosomes is 
reduced to one half, so that the spore has the same number of 
chromosomes as the sexual plant to which it gives rise. The alter¬ 
nation in these Algae is thus antithetic in the sharp cytological 
differentiation between the two generations, but indications are 
wanting to show that the sporophyte has here had the history of 
gradual interpolation, which I take to be the essential of the anti¬ 
thetic theory as applied by Bower and Celakovsky to the origin of 
the sporophyte in archegoniate plants. The body of the sporo¬ 
phyte appears indeed to be strictly homologous with that of the 
sexual plant. 
In the Liverworts, Mosses and the various groups of the 
Vascular Cryptogams we also find two generations, which regularly 
alternate. The one reproduces sexually and its cells have the 
half number of chromsomes: the other reproduces by spores and 
its cells have the double number of chromosomes. The two germ- 
cells, as in the Algae referred to, are the spore and the fertilised 
egg, but here their products are very different, not merely in their 
reproductive organs but in the form and structure of their bodies. 
In seeking for an explanation of the great differences between 
the alternating generations which are characteristic of the Bryo- 
phyta and Pteridophyta it is evident that there are two possible 
views :— 
(a) That the germ-cells are so different that they necessarily 
give rise to bodies of different structure. 
(bj That the two germ cells alike represent specific cells of 
the plant, but that the conditions under which they develop are so 
different that two very unlike bodies result. 
If we adopt the first view we have to regard the spore and 
fertilised egg as two profoundly different modifications of the 
specific cell. We are practically dealing with two specific cells 
(one for the sexual and one for the asexual generation), each of 
which has latent within it the characters of the other generation. 
On the second view the germ-cells would be essentially alike, the 
