LIFE HISTORY OF A FERN 



169 



chance, to lie upside down. The dorsal surface would 

 then be the under surface, and the ventral surface the 

 upper one. Organisms or organs having two such surfaces 

 clearly distinguishable are said to have dorso-ventral 

 differentiation. Among many other structures thus dif- 

 ferentiated are foliage-leaves, sporophylls, man, fishes, and 

 other animals. 



154. Reproductive Organs : Archegonia.^Examination 

 of the ventral surface of a mature prothallus with a lens 



Fig. 127. — Archegonia of a fern {Adiantum). A, young archegonium; 

 B, mature; C, top view, showing terminal cells of the four rows of wall 

 cells; V, wall of venter; e, egg; v.cx, ventral canal-cell; n.c, neck-canal; 

 sp, sperms entering the neck-canal. A and B in longitudinal section. 



will reveal near the notch and on the cushion, several 

 tiny flask-shaped bodies, the archegonia. Each arche- 

 gonium consists of a wall, one cell thick, and contents 

 (Fig. 127). The neck projects away from the surface, 

 and is usually slightly curved, while the remainder, the 

 venter, is imbedded in the tissue of the cushion. As the 



