EMBRYOGENESIS IN THE BRYOPHYTA 71 



transverse, and sometimes very markedly so (McNaught, 1929). A 

 typical quadrant stage was not observed in this species. Two primary 

 transverse walls yield a short filamentous embryo of three cells, which 

 develop respectively into the foot, seta and capsule. In Marchantia sp. 

 from Peru, Heberlein (1929) observed the first division of the zygote to 

 be transverse or somewhat oblique. After a typical quadrant stage the 

 embryo broadens out, i.e. enlarges at right-angles to its axis. According 

 to the shape of the embryo, the next walls may be formed in somewhat 

 different positions, the successive divisions resulting in a spherical mass 

 of small cells. At this stage the cells are all much alike, but later those 

 in the distal half become more densely protoplasmic, i.e. in preparation 

 for the formation of the capsule. That species of the same genus do not 

 necessarily show the same type of early development is demonstrated 

 by the foregoing observations. Again, in two species of CyatJwdium, 

 C. cavernarum has a filamentous type of embryo (Lang, 1905) while 

 C.foetidissimum has an octant type. 



Lastly, it may be noted that, following the fertilisation of the ovum, 

 the venter wall and the tissues round the base of the archegonium are 

 stimulated to a considerable growth development, the results being 

 seen in the formation of the enveloping calyptra and the pseudoperianth. 

 These developments are indicative of the active outward diffusion of 

 growth-regulating substances from the zygote and developing embryo 

 {see also p. 73, Fig. 16). 



HEPATICAE — SPHAEROCARPALES 



In the Sphaerocarpales the zygote is divided by a transverse wall 

 into two approximately equal parts. Each of these cells is again 

 divided by a transverse wall (or walls) before any vertical walls appear. 

 The young embryo thus consists of a simple row of cells as in some 

 Marchantiales, the first wall defining the epibasal and hypobasal 

 regions. In the hypobasal region, however, growth is considerably 

 less vigorous, the result being a pear-shaped embryo with a narrow 

 base. Fig. 14k. On further development the hypobasal region forms a 

 short, narrow seta and a somewhat distended parenchymatous foot. 

 Thus, although the early embryogeny in Sphaerocarpus is typically 

 filamentous, a not untypical hepatic sporophyte is eventually formed. 

 It is apparent that the growth rate at different points along the axis is 

 very different and it seems unlikely that this can be explained in terms 

 of metabolic gradients having their source in the gametophyte tissue, 

 though these are no doubt involved. The morphological development 

 is a distinctive and regulated one and, in order to explain it, it is neces- 

 sary to account for the differential distribution of metabolites in the 

 basal, median and distal regions. Genetic^] factors are, no doubt, of 



