THE PTERIDOPHYTA : LYCOPSIDA, ETC. 



585 



prothalli are consequently minute, though the female is less reduced than 



the male. 



The contents of the microspore divide into two cells of very unequal 

 size. This is accomplished before the spores are shed. The smaller cell is 

 called the prothallial cell and takes no further part in development (Fig. 

 594). It appears to be all that represents the vegetative part of the prothallus. 



Fig. 594. — Selagiuella apoda. Development of the male gametophyte. A, Section of 

 the microspore showing division of contents to form lenticular prothallial cell, 

 the remainder constituting the antheridial cell. B, First division of the nucleus 

 of the antheridial cell. C, Differentiation of the antheridial wall cells. D and E, 

 Differentiation of the antherozoid mother cells. F, Dehiscence of the microspore 

 to release the antherozoids. All cell walls have disappeared. {After Lyon.) 



The larger cell, which is called the antheridial cell, continues to divide until 

 a group of twelve cells is formed. Of these, eight form a wall enclosing a 

 group of four central cells. Whilst these changes have been going on within 

 the spore, its outer wall bursts at the triradiate ridge, so that the outer end 

 of the developing tissue is only covered by a thin cellulose wall. The central 



