PTERIDOPHYTA. 199 



central portion, which encloses the oosphere, is always embedded 

 in the tissue of the prothallium, so that the neck, which is formed 

 of 4 rows of cells, projects above the surface (Figs. 201 3 , 222 h). 

 The development of the archegonium in a Fern is seen in the 

 accompanying figure (Fig. 201). The archegoniumis developed from 

 a surface cell, which divides into three cells by two walls in 

 a direction parallel to the surface of the prothallium (Fig. 201). 

 The most internal cell becomes the ventral portion of the arche- 

 gonium. The external one (6) divides perpendicularly to the sur- 

 face of the prothallium into four cells, which again divide parallel 

 to the surface and form the neck (6, in 2 and 3). The interme- 

 diate cell projects upwards into the neck and divides into two, 



FIG. 201. Pleris serrulata. Development of archegonia. 



the lower one, after the separation of the ventral canal-cell, becom- 

 ing the oosphere, arid the upper one the neck-canal-cell (c, in 2 and 3). 

 . As in the Mosses, the divisional walls of the neck-canal-cells 

 become mucilaginous, causing the rupture of the neck of the 

 archegonium. Fertilisation takes place as in the Mosses, and the 

 passage of the spermatozoids, along the neck, to the oosphere, 

 has been observed. Water (rain or dew) is similarly necessary for 

 the movements of the spermatozoids, and hence for fertilisation. 

 The other classes of the Division chiefly deviate from the Ferns 

 in having the archegonium sunk deeper into the prothallium, and 

 the neck reduced in length (compare Fig. 201 with Figs. 216, 222, 

 235, 236). 



