XXXI POLYPLAST AND PHYLLULA 427 



plane parallel to the neck of the ovary, forming two cells, an 

 anterior nearest the growing or distal end of the prothallus, 

 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 consist- 

 ing 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. 405). In the fern the later stages are more complex. 

 One of the upper anterior cells remains undeveloped, the 

 other (Fig, iii, 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 Xhefoot {ft), 

 which becomes embedded in the prothallus, 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, i.e., 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. 422) : 

 division of this cell goes on very rapidly, and ^primary root 

 is produced 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, called the 

 cotyledon {ct), which soon begins to grow upwards towards 

 the light. 



