xxix POLYPLAST AND PHYLLULA 359 



and a posterior towards its proximal end. Each of these 

 divides again by a plane at right angles to the first, there 

 being now an upper and a lower anterior, and an upper and 

 a lower posterior cell : the lower in each case being that 

 towards the downwardly directed neck of the ovary. Each 

 of the four cells undergoes fission, the embryo then consisting 

 of eight cells, two upper anterior (right and left), two lower 

 anterior, two upper posterior, and two lower posterior. We 

 thus get a multicellular but undifferentiated stage, the 

 polyplast. 



It will be remembered that in mosses the polyplast forms 

 an apical cell, and develops directly into the sporogonium 

 (p. 337). In the fern the later stages are more complex. 

 One of the upper anterior cells remains undeveloped, the 

 other (Fig. 83, i and K, st) takes on the form of a wedge- 

 shaped apical cell, and, dividing in the usual way, forms a 

 structure like the apex of the fern-stem, of which it is in fact 

 the rudiment. The two upper posterior cells divide and 

 subdivide, and form a multicellular mass called the/w/(//), 

 which becomes embedded in the prothailus, and serves the 

 growing embryo for the absorption of nutriment. One of the 

 lower posterior cells remains undeveloped, the other (rt) 

 takes on the form of the apical cell of a root, /.*., of a wedge- 

 shaped cell, which not only produces three sets of segmental 

 cells from its sides but also cap-cells from its base (p. 354) : 

 division of this cell goes on very rapidly, and a root is pro- 

 duced which at once grows downwards into the soil. Finally 

 the two lower anterior cells undergo rapid fission, and 

 develop into the first leaf of the embryo or cotyledon (ct\ 

 which soon begins to grow upwards towards the light. 



Thus at a comparatively early stage of its development 

 the fern-embryo has attained a degree of differentiation far 

 beyond anything which occurs in the moss-embryo. The 



