XIV] GAMETANGIA AND SPORANGIA 289 



divides by a periclinal wall, the inner dividing again, so that three super- 

 imposed cells result. Of these the outermost forms the neck, dividing by 

 crossed cleavages into four cells : each of these by subsequent divisions, 

 which vary in number in different Ferns, gives rise to one row of cells of 

 the neck. The innermost or basal cell takes no further direct part, and may 

 perhaps represent a sterilised part at the base of the central series (see 

 below). The middle cell of the series divides again periclinally to form 

 the canal-cell, and the central cell. These each divide again periclinally: 

 in the former the division is often incomplete, and confined to the nucleus: 

 in the central cell the last division gives origin to the ventral-canal-cell and 

 to the ovum (Fig. 278, c). 



Comparison of Gametangia with Sporangia 



As seen in relatively advanced types of Ferns such as Dryopteris 

 the antheridia and archegonia appear to differ clearly from one another, 

 both in form and in their contents (Figs. 20, 21). But in certain of those 

 Ferns which on the sum of their characters may be held as relatively 

 primitive their form is less distinct, though still the contents differ. For 

 instance in Marattia, where both are sunk in the tissue of the prothallus, 

 not only does each spring from a single deeply sunk superficial cell of cubical 

 form, and maintain that form to maturity, but the segmentation of the parent 

 cell by a periclinal wall gives rise to an inner and an outer cell (Fig. 278, b, d). 

 The latter produces on the one hand the protective antheridial wall, and on 

 the other the neck of the archegonium : the former is the parent cell of the 

 gametes, giving rise respectively to the spermatocytes, or to the ovum with 

 its attendant canal-cell and ventral-canal-cell, together with the basal cell. 

 Such a degree of similarity raises the question of homology of the game- 

 tangia, in the sense that the two types may have originated from a single 

 type of gametangium. Organs intermediate in character between antheridia 

 and archegonia have not been recorded for the Pteridophyta, and abnor- 

 malities in their archegonia are rare. But in the Bryophyta examples have 

 been described and figured (von Goebel, Organographies i, p. 129, footnote). 

 The most notable are those archegonia of Mnium which have been seen to 

 contain not only an &^^ and ventral canal-cell, but sometimes several eggs, 

 while others show masses of spermatocytes, and these may be both above and 

 below the ovum (Fig. 280). Such a structure appears to be in fact a bisexual 

 organ, having both antheridial and archegonial characters. On these facts 

 the view is based that the antheridium and archegonium of Bryophytes are 

 probably homologous organs, derived from a primitively uniform game- 

 tangium. In the case of the archegonium extensive sterilisation of the 

 reproductive cells has led finally to the survival of only one functionally 

 B. 19 



