Alternation of Generations and Ontogeny. 
21 i 
The question of the relation of the two generations in Dictyota 
has now to be considered. This point is of great importance, for 
it was a comparison between Dictyota on the one hand (with its 
somatically similar generations, and its germ-cells developing 
under similar conditions), and, on the other hand, such forms as 
ferns (with their dissimilar generation and germ-cells developing 
under different conditions), which appears to have first led to the 
suggestion of the ontogenetic theory. 
In Dictyota we have the generations with similar bodies, but 
bearing two different types of reproductive organs. The question 
naturally arises how is the orderly succession of the two generations 
brought about. Why does the tetraspore always give origin to 
the form bearing sexual organs and the egg give origin to a plant¬ 
bearing sporangia, when the two germ-cells appear to develop 
under identical conditions ? 
There are two possible views: — 
(a) . That the germ-cells behave differently because of 
their chromosome number, haploid and diploid respectively. 
(b) . That the germ-cells behave differently because of 
their different position in the life-cycle, i.e., that in their development 
they have received different tendencies. 
The hypothesis that the difference in the mode of reproduction 
is necessarily associated with cytological differences of chromosome- 
number is, of course, attractive; but just as the view of the relation 
of somatic differences and chromosome-number has had to be given 
up so the evidence is clearly against this view. The serious 
objections to such a view are based on general considerations and on 
a certain amount of definite evidence. It is now generally accepted 
that the diploid cell of any animal or plant contains a double set of 
determinants (pangens), and every haploid cell a single set of 
determinants. In the case of Dictyota then, every diploid cell will 
have a double set of somatic, sexual and tetraspore characters, and 
every haploid a single set of somatic, sexual and tetraspore characters. 
On general grounds, therefore, the formation of tetraspore-bearing 
plants by diploid cells alone is not to be explained by their cytological 
characters, as even the haploid cells possess the tetraspore 
determinants. 
There is also definite evidence against the necessary connexion 
of the haploid and diploid character with the type of reproduction. 
In ferns and mosses, diploid gametophytes can bear eggs and 
