EMBRYOGENESIS IN THE BRYOPHYTA 67 



quadrant and filamentous types of development have been recorded 

 (Cavers, 1904; Meyer, 1929). 



The Quadrant Type. In some species such as Targionia hypophylla. 

 Fig. 14f, and Conocephalum conicum, as described by Meyer (1929), 

 the zygote is unequally divided into a large epibasal and a small hypo- 

 basal cell: on further development the former will give rise to the 

 capsule and seta, the latter to the foot or to the basal region of the seta 

 and the foot. In quadrant types, the embryo is next divided by longi- 

 tudinal walls at right-angles to the first wall and to each other, so that 

 it now consists of an octant of approximately equivalent cells. Although 

 we know very little about these cells, their subsequent developments 

 suggest that even at this early stage they are not all alike physiologically. 

 The cells of the hypobasal region function as the absorbing system of 

 the embryo from the outset, the individual cells tending towards a 

 parenchymatous type of development. The epibasal cells, on the other 

 hand, remain meristematic and constitute the region of active growth 

 and differentiation. After the octant stage the embryo begins to 

 elongate, the next divisions in both epibasal and hypobasal regions 

 being similar, and the segmentation pattern developed being of con- 

 siderable regularity. The divisions in the two regions do not, however, 

 proceed at the same rate, nor are they duplicates of each other. The 

 new walls are typically laid down at right-angles to the curved outer 

 wall and adjoin the internal walls as shown in Fig. 14c, g. Other 

 transverse walls are also laid down. Then, at a characteristic stage of 

 development, periclinal walls are formed and these divide the upper 

 region of the embryo into the amphithecium and endothecium, the 

 former giving rise to the capsule wall, the latter to the archesporium. 

 It is about this stage of development that we might perhaps regard the 

 embryogeny proper as coming to an end; for on further growth and 

 development the sporophyte becomes differentiated as a distal spore- 

 containing capsule with a short seta and a basal haustorial foot. In 

 these organisms there is, in fact, no sustained, adult vegetative phase, 

 antecedent to the spore-producing phase, as in pteridophytes. 



The sporophyte in the Marchantiales, then, is characterised by a 

 very regular initial embryonic development which soon merges, or is 

 transformed, into the mature spore-bearing capsule. As in the develop- 

 ment of certain animal embryos, the fates of particular regions of the 

 developing germ are determined from an early stage. In that no steady 

 state of vegetative development is established, but rather that there is 

 an early transition from the embryonic to the sporogenous phase, we 

 may assume either that the quantity and quality of the nutrients being 

 received by the embryo from the parent gametophyte vary in a 

 characteristic manner during development, due perhaps to ageing, to 



