The ARCHEGONIA are more uniform throughout the entire Division, and more closely resemble those of the Mosses. They are, as in the previous Division, principally flask-shaped; but the 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 archegonium is 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 archegonium. The external one (_b_) divides perpendicularly to the surface of the prothallium into four cells, which again divide parallel to the surface and form the neck (_b_, in 2 and 3). The intermediate cell projects upwards into the neck and divides into two, the lower one, after the separation of the ventral canal-cell, becoming the _oosphere_, and the upper one the _neck-canal-cell_ (_c_, in 2 and 3). [Illustration: FIG. 201.--_Pteris serrulata._ Development of archegonia.] 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). According to the nature of the spores, the three classes of the Vascular Cryptogams are each divided into isosporous and heterosporous groups. I. The =isosporous= Vascular Cryptogams have _only one kind of spore_. The prothallium developed from this is in some cases monœcious, bearing both antheridia and archegonia; but in others there is a distinct tendency for each prothallium to bear only antheridia or archegonia (diœcious)--true Ferns and _Lycopodium_. In _Equisetum_ there is only one kind of spore, but two kinds of prothallia are developed, one of which bears only antheridia (male), the other only archegonia (female); but the one that bears antheridia may be transformed into the one that bears archegonia and vice versa. II. In the higher group, =heterosporous= Vascular Cryptogams (_Selaginella_ and _Isoëtes_, etc.), there are two distinct kinds of spores, the _small_, microspores, and the _large_, macrospores. The _microspores_ are male, and produce prothallia which bear only antheridia. The _macrospores_ are female, and produce prothallia which bear only archegonia.