XV] 



PRIMARY SEGMENTATION 



301 



composed of epibasal and hypobasal tiers, each consisting of four cells and 

 separated by a septum which is called the basal wall. It appears to be the 

 rule thdiX. for all fully investigated embryos of PteridopJiytes the position of the 

 apex of the axis is constant in relation to this basal wall. Whatever may be the 

 other fluctuations of form of the embryo, or of the relations of the other 

 parts, tJie apex of the axis originates as nearly as possible to the centre of the 

 epibasal hemisphere of the embryo, that is, in close relation to the intersection 

 of its octant walls. This being so, it is clear that the polarity of the embryo 

 is determined structurally at the time of the first segmentation of the zygote: 

 or objectively prefigured by the position of the nuclear spindle in that first 

 segmentation. This generalisation was illustrated in Chapter XLII of the Land 

 Flora by reference to the various types of embryogeny then known, and 

 all those which have been fully described since conform to it. 



II III 



^ ^fe 



Fig. 284. Diagrams illustrating the segmentation of embryos. I = where a 

 suspensor is formed, which is cut off by the first wall, /, /: the suspensor 

 is cross-hatched ; B, i9=the basal wall, separating the hypobasal hemi- 

 sphere (dotted) from the epibasal (clear). II = the same seen from above, 

 X marking the pole. III=an embryo where no suspensor is formed, 

 and the segmentation resembles that in the embryonic cell where the 

 suspensor is present; the lettering corresponds: x, y indicate the 

 polarity. Each hemisphere divides into four Ijy quadrant walls {QQ in 

 II), and octant walls {0, o). 



A general conception of the embryo of Pteridophyta which folloivs from 

 comparison of them all, zvhether with or without suspensor, is that it is a body 

 possessed of polarity from the very first : that in form it is at first commonly 

 a more or less spindle-shaped body, composed of two or more component tiers. 

 It is in fact based on the ultimate type of a transversely septate filament. This 

 applies also for Seed-Plants. It is most apparent in those embryos which 

 retain the suspensor. In them the whole product of the zygote consists at 

 an early stage of a simple row of cells, which may be more or less abbreviated ; 

 the cells are liable to lateral distension, with or without further cell-divisions. 

 Upon the primordial spindle thus formed appendages may originate, but 

 with latitude of difference in their number and in their proportion. The 

 determination of their dimensions and time of appearance may be correlated 

 with circumstances. For instance, in mycorhizic Ferns, such as Botrychium 

 Lunaria and Ophioglossum vulgatum, the root is hurried quickly forward. 



