i68 MORPHOLOGY 



but among the Polypodiaceae usually only the nucleus divides, resulting 

 in a single binucleate neck canal cell. The primary ventral cell divides, 

 as usual, into the ventral canal cell and the egg (fig. 393, c). In passing 

 from Marattiaceae to Polypodiaceae, one passes from broad canal cells 

 to small ones, and from two distinct neck canal cells to the elimination of 

 the wall (binucleate condition). There is thus a gradual disappearance 



a 



FIG. 393. Archegonium of a fern: a, young stage, showing two neck cells, central 

 cell (shaded), and basal cell, all from single initial; b, older stage, showing further 

 development of neck and enlargement of central cell preparatory to forming the axial 

 row ; c, neck further developed and axial row complete (binucleate neck canal cell, 

 ventral canal cell, and egg); d, mature archegonium, with neck complete, neck canal 

 cell disorganized, ventral canal cell breaking down, and egg rounded off. 



of the neck canal cells, a process which is completed when seed plants 

 are reached. 



Fertilization. In fertilization the sperm enters the neck of the 

 archegonium, ciliated end first; and then the ciliated beak ceases 

 to function. As the sperm enters the cytoplasm of the egg, the cyto- 

 plasmic sheath of the sperm, including the beak, is often left behind, 

 and the male nucleus moves alone through the egg cytoplasm to the fe- 

 male nucleus; but in some cases the whole sperm has been observed 

 within the egg nucleus. 



Embryo. In the development of the embryo, the four great body 

 regions are said to be differentiated at the quadrant stage ; but in passing 

 from Marattiaceae to Polypodiaceae the quadrants are directed dif- 

 ferently. Among Marattiaceae the first division of the egg is transverse 

 to the long axis of the archegonium (fig. 394), by far the most common 

 plane. The current account is that this first division differentiates the 

 shoot (stem and leaf) from the foot and root, the former being repre- 

 sented by the inner cell, the latter by the outer; and that at the quadrant 

 stage the two outer (ventrally directed) quadrants develop the foot and 



