Alternation of Generations based on Ontogeny. 7 
relation to the soil, water, light, etc. The fertilised egg, on the 
other hand, develops in relation to the body of the sexual genera¬ 
tion. It thus develops, under profoundly different conditions 
from the spore, firstly in that it is removed from all the influences 
acting on the spore, and secondly in that it is exposed to a new set of 
nutritive and correlative influences proceeding from the maternal 
body. We know practically nothing of the nature of these influences, 
but we can appreciate their existence and importance. When we 
are able to change the conditions of development of an organism 
experimentally we are able to effect considerable changes in its 
form and structure, but in no case are we in a position to alter the 
conditions so fundamentally as is done in the case of a germ-cell 
developing within the tissues of the parent organism instead of as 
a free cell. 
In the case of the spore-producing generation of Bryophyta 
this removal from external influences and exposure to the maternal 
influence of the sexual generation last practically throughout the 
development of the sporogonium, In the Pteridophyta the sexual 
generation ultimately becomes free from the prothallus. In all cases 
however the development is initiated and has advanced to the 
establishment of the various organs of the sporophyte under the 
maternal influence. We are justified in assuming from some 
particular cases which have been studied that each stage in the 
ontogeny is determined by the preceding stage. If therefore, as in 
the Pteridophyta, the first steps of development have taken place 
under the influence of the prothallus, the influence of the preformed 
parts of the young sporophyte may be legitimately assumed to 
exercise a “ formative induction ” on the further course of its 
development. 
It may be noted in passing that these considerations on the 
influence of the enclosing parent generation on the enclosed stage, 
which is at the same time removed from the direct influence of the 
external environment, are capable, to some extent, of application 
also to the converse relation of the asexual and sexual generations 
in the seed-plants. Here it is the gametophyte which develops 
enclosed within the body of the sporophyte. 
In both these cases of parental association the direct influence 
of the retained organism on the part enclosing it must also be 
considered. The comparison with gall formation, due to the influence 
of an insect egg or larva, is here very suggestive. The development 
of the calyptra enclosing the sporogonium of Aneura or Anthoceros 
for instance may be regarded as a sort of gall due to the presence 
