428 FERNS LESS. 



Thus at a comparatively early stage of its development 

 the fern -embryo has attained a degree of differentiation far 

 beyond anything which occurs in the moss-embryo. The 

 scarcely differentiated polyplast has passed into a stage 

 which may be called the phyllula, distinguished by the 

 possession of those two characteristic organs of the higher 

 plants, the leaf and root. 



Notice how early in development the essential features of 

 animal or plant manifest themselves. In Polygordius the 

 polyplast is succeeded by a gastrula distinguished by the 

 possession of a digestive cavity : in the fern no such cavity 

 is formed, but the polyplast is succeeded by a stage dis- 

 tinguished by the possession of a leaf and root. In the 

 one case the characteristic organ for holozoic, in the other 

 the characteristic organs for holophytic nutrition make their 

 appearance, and so mark the embryo at once as animal 

 or plant. We may say then that while the oosperm and 

 the polyplast stages of the embryo are common to the 

 higher plants and the higher animals, the correspond- 

 ence goes no further, the next step being the formation 

 in the animal of an enteron, in the plant of a leaf and 

 root. In other words the phyllula is the correlative of the 

 gastrula. 



The cotyledon increases rapidly in size, and emerges 

 between the lobes of the kidney-shaped prothallus (l) : the 

 root at the same time grows to a considerable length, the 

 result being that the phyllula becomes a very obvious 

 structure in close connection with the prothallus, and indeed 

 appearing to be part of it. The two arc actually, however, 

 quite distinct, their union depending merely upon the fact 

 that the foot of the phyllula is embedded in the tissue of 

 the prothallus like a root in the soil. Hence the phyllula 

 is related to the prothallus in precisely the same way as the 



