Alternation of Generations and Ontogeny. 215 
complete metamorphosis. It is not necessary to assume that there 
are two completely separate sets of characters for the prothallus 
and sporophyte, some characters may be in common, as in insects. 
A certain similarity beween gametophyte and sporophytic 
characters gives no support to the ontogentic view. A sim)ar\jty is 
strikingly shown in the two body-forms of insects and of 
Foraminifera. 
It is assumed in the ontogenetic theory under discussion that 
in the spore which develops on soil certain sporophytic characters 
do not find the conditions suitable for their development and so 
never appear. When we pass to a form like Selaginella some 
further hypothesis seems to be required to explain why some or all 
the sporophytic characters do not appear in the developing micro¬ 
spores and megaspores, for the conditions in the sporangium may 
be considered as fairly similar to those in the archegonium. 
There is one further criticism which may be made, and that is 
the question of the importance of the retained position of the egg. 
It is suggested “that once the dependent position of the diploid 
generation was established profound and probably sudden changes 
might be expected to follow, resulting in the difference in body-form 
between sporophyte and gametophyte.” The reason for this view is 
not quite clear. The egg-cell does not appear to be in such an ex¬ 
tremely sensitive condition, for when Heape transferred the fertilized 
egg from the uterus of one kind of rabbit to another, development 
was normal. Furthermore, zoologists do not appear to attach any 
special significance to the question whether the egg passes its early 
stages of development free, or enclosed in the maternal tissues. 
They do not necessarily find striking differences in the mature 
organisms as a result of the difference of environment in the early 
stages of egg development. In Zoogonoides viviparus , for example, 
we have a form which belongs to the same family as Distomum 
hepaticum (the liver-fluke) already mentioned. The egg of the first- 
named form develops within the maternal tissue, and yet the pro* 
duct is very similar to that arising from the free egg of Distomum . 1 
The question of experimental work is obviously of importance, but 
it would appear, unfortunately, that very little is to be hoped for in 
this direction ; for not only will critical experiments be of great 
technical difficulty, but even if experiments are successful the 
interpretation of them will be exceedingly difficult. The experiments 
1 I am indebted to Miss Lebour of the University of Leeds for 
this case. 
