Discussion on “ Alternation of Generations .'' 1 107 
[The slides shown included photographs of the haploid and 
diploid individuals in Dictyota and Polysiphonici ; Coleoclicete, the 
fruit-body of which it was suggested might be regarded as a mass 
of haploid cells, influenced by retention to simulate a sporogonium ; 
Ocdogoniurn, with no second generation but the division of the zygote 
into four zoospores ; Bryophyta, in which the influence of the 
enclosing generation lasts throughout the development of the sporo¬ 
gonium ; apospory in Mosses and in Anthoceros, in which cells of 
the sporophyte more or less isolated from their connections and 
placed under the conditions of a germinating spore, behave in their 
further growth like the latter. 
In Vascular Cryptogams the retention of the spore-bearing in 
the enclosing sexual generation ceases after a time, but not until 
the main organs of the plant have been laid down, and a state of 
formative induction of the already formed parts on the future 
development may be supposed to be established. Examples of the 
great phyla were considered, and it was suggested that the leaves 
of Lycopods and Equisetum might correspond to the lobes borne 
on the prothallus, while in the Ferns the comparison was rather 
between the early leaves and a whole branch of the prothallus. 
Correspondences between the sporangia and the antheridia in the 
various groups were also indicated. The protocorm of Lycopodium 
cernuum and other species, while not necessarily a primitive organ, 
was considered an expression of the capacity of the diploid embryo, 
brought partly under the conditions of life of the prothallus, to take 
on resemblances to the latter. 
The remarkable body of evidence which the facts of apospory 
and apogamy supplied by the Ferns was briefly reviewed. Though 
apospory and apogamy probably represent nothing that has formed 
a stage in descent, they afford valuable data as to the causes of the 
ontogeny.] 
I have avoided till now entering into the bearing of the different 
conditions of ontogenetic development of the two generations in the 
life-history upon the antithetic and homologous theories of alternation. 
I do not intend to do so fully now; time is limited, and this will be 
done better by other speakers. I would point out, however, that 
the facts we have learnt in recent years, as to the nuclear difference 
between the two generations in archegoniate plants and in certain 
Algse, have tended to modify both the great rival hypotheses and 
made it more difficult to define their essential characteristics. This 
is a subject upon which the discussion may throw more light. 
From one point of view an alternation such as is seen in Dictyota 
may be regarded as antithetic, and from another as homologous. 
We have an antithesis in cytological characters, but such an 
agreement in the organisation of the two generations that I should 
regard them as homologous. This plant also suggests that the 
diploid generation need not in any way have been the result of 
gradual interpolation and progressive sterilisation, which may be 
regarded historically as the central idea of the antithetic theory. 
On the other hand I do not see any evidence for the spore-bearing 
form being the result of gradual modification of a sexual individual, 
which may be taken as the main idea of the older homologous 
theory. I see no reason why, if reduction did not take place in the 
zygote, we might not get suddenly from the latter, a generation 
