THE PTERIDOPHYTA : LYCOPSIDA, ETC. 631 



to the external surface. The upper cell divides two or three times to form 

 a series of jacket cells, one of which forms a three-sided operculum, whilst 



Antheridium 



Fig. 640. — Equisetiim orvense. Prothallus with 

 antheridia containing antherozoids. 



the lower cell, after numerous divisions, forms the antherozoid mother cells. 

 From each of these cells a single antherozoid is formed. This antherozoid 



Fig. 641. — Equisetiim arveme. Mature 

 antherozoid. {After Sharp.) 



consists of an homogeneous, spirally coiled body derived almost entirely 

 from the nucleus of the mother cell, whilst its numerous apical flagella 



