ORIGINS OF EMBRYONIC PATTERNS 649 



pattern. The development of Selaginella affords an interesting example 

 of these changes. The megaspore has an apicobasal axis; and development 

 of the apparently radially symmetrical megagametophyte is at first lim- 

 ited to the apical region, the basal region which contains much starch 

 becoming cellular only later. The apical region of the gametophyte in 

 which archegonia develop is exposed by rupture of the spore wall over it, 

 but most of the gametophyte remains within the spore wall (Fig. 207, A). 

 Archegonial and gametophyte axes are in the same direction. The first 

 division of the zygote in S. martensii is transverse to these axes, as indi- 

 cated in the two-cell stage in Figure 207, A (Bruchmann, 1909). From 

 the cell next to the neck of the archegonium the suspensor develops and 

 elongates, pushing the embryo deep into the gametophyte tissue; the 

 other cell gives rise to the embryo. This embryonic cell divides vertically, 

 and each resulting cell again vertically. Two of the embryonic cells are 

 shown in Figure 207, B. From one of the four cells resulting from the 

 first two divisions the apical cell of the stem is separated by a diagonal 

 wall (a of Fig. 207, C). As development progresses, the embryonic axis 

 gradually bends to one side (Fig. 207, D) and finally more or less toward 

 the apical region of the gametophyte, the foot develops toward the base 

 of the gametophyte, and the rhizophore opposite the stem (Fig. 207, E). 

 At this stage the embryo with suspensor and foot is a bilaterally sym- 

 metrical individual, but how change in direction of the embryonic axis 

 which results in definition of a median plane is determined seems not to 

 be known. The gametophyte is apparently radial; and, according to 

 Bruchmann, gravity is not concerned. Conceivably the change in form 

 from that of Figure 207, C, to that of Figure 207, E, may result from a 

 reaction of the stem axis to an apicobasal differential in the gametophyte, 

 or it may be associated with development of the large foot. But whatever 

 the factors concerned in these changes in form and direction of growth, 

 it seems evident that the pattern of early development shows a definite 

 and constant relation to archegonial or gametophyte pattern or to both. 

 The gametophyte of Isoetes is somewhat similar to that of Selaginella, 

 but relations of embryonic regions are apparently different. The first di- 

 vision of the zygote is transverse or inclined to the archegonial axis, 

 the cell toward the base becoming foot, the other embryo, suspensor 

 being absent. According to this account, the embryonic axis is oppo- 

 site in direction in relation to its gametophytic environment to that 

 of Selaginella. Perhaps early determination of the stem axis in Selaginella 

 and apparently much later determination in Isoetes are concerned in this 



