CH.xv] EMBRYOLOGICAL METHOD 299 



Probably a middle position will ultimately be found to give conclusions 

 nearest to the truth. We shall therefore be prepared in studying the em- 

 bryology of the Filicales to recognise underlying features, inherited possibly 

 from a remote ancestry ; and to see how the working out of the ancestral 

 type may have been modified in any given case by biological conditions 

 affecting the embryo during its development. Doubtless opinion will vary 

 according as greater weight is attached to the first or the second factor. 

 This is indeed a legitimate field for discussion as part of the embryological 

 problem, when once the facts have been ascertained. In view of the many 

 recent additions to knowledge it may be held that, as regards the embryology 

 of the Filicales, the detailed facts are so far before us that a reasonable basis 

 for forming such opinions already exists. 



Embryological argument will necessarily turn in great measure on the 

 segmental cleavages in early states of the embryo. To-day these are taken 

 as indicators of the direction or localisation of growth. Such growth being 

 unequal at different points and in different directions, it will result in the 

 adoption of certain features of form by the embryo ; and these should be 

 considered in close relation to the cell-cleavages. In the nature of the case 

 the study of embryology thus becomes a highly technical branch of the 

 subject, and one in which it is specially necessary to discriminate between 

 what is essential and recurrent in many types or in all, and what is of in- 

 constant occurrence, and on that account to be held as accessory. 



The embryo of an ordinary Leptosporangiate Fern has been described 

 in Chapter I, and stages in its development are shown in median plane 

 of section in Figs. 24 and 25, for the case of Adiantum. Like other embry- 

 onic tissues of the more advanced Ferns the embryo shows very regular 

 segmentation, and it is possible from an early stage of development to refer the 

 several parts of the young plant, viz. the stem, leaf, root, and foot, to definite 

 segments cut off by the first cell-cleavages in the zygote. Such reference gave 

 the opportunity for the facile conclusion that there is a causal connection 

 between cleavages and the initiation of the several organs. The facts of 

 segmentation seen in the Fern-embryo and in the young sporogonium 

 of a Moss have been used, more than perhaps any other examples, as a 

 foundation for an elaborate theory of embryology based upon cell-cleavages, 

 or as von Goebel has styled it, a sort of " Theory of Mosaics " {I.e. p. 978). 

 The elucidation of the facts coincided in time and in tendency with advances 

 in the knowledge of the genesis of tissues from apical meristems, as demon- 

 strated in particular cases by Hanstein. These appeared also to accord with 

 the development of the science of animal embryology, in which the theory 

 of germinal layers took a firm hold. Botanists allowed themselves to be 

 influenced by these results, and many of them accepted the view that the 

 segmentation at the apex of the stem determined the disposition of tissues. 



