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.c.c, 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 
