DEVELOPMENT OF THE OSTRICH FERN. 



23 



elongating with the prothallium, forms a sort of mid-rib leaving the remainder of the 

 prothallium a single layer of cells. In these large prothallia the edges are usually decid- 

 edly wavy. 



The motlier-cell of the arcliegonium (PI. 5, fig. 3) is not easily detected at first, resem- 

 bling in shape and size the other young cells in the vicinity. Careful examination, how- 

 ever, usually makes it distinguishable. Its shape, like that of many of the surrounding- 

 cells, is that of a truncated pyramid, the base being at the surface of the prothallium. 

 Viewed from above it appears nearly square. The contents are quite different from those 

 of the neighboring cells. In the latter, the protoplasm together with the chlorophyll-o-ran- 

 ules and nucleus is coufiued to the parietal parts of the cells, the center containing only 

 cell-sap; in the mother-cell of the archegonium, on the contrary, the protoplasm appears 

 to be moredense occupying nearly or quite all of the cell. The chlorophyll granules are 

 smaller,^less numerous and distinct, while the nucleus is not only larger but nearly cen- 

 tral in position instead of lying near the wall. 



The first step in the further development of the cell is a slight elevation of the outer 

 wall above the level of the neighboring cells, the upper part of the cell becoming usually 

 more transparent. ]^ext, the cell becomes divided into two unequal parts by a transverse 

 wall; the lower cell undergoes no further change beyond simple division, taking no 

 further part in the formation of the archegonium, but acting in all respects like the veo-- 

 etative cells of the prothallium. The upper and larger cell next divides by another wall 

 parallel to the first, but more or less convex, the outer of the two cells thus formed con- 

 taining less protoplasm than the lower and forming the beginning of the neck; the lower, 

 filled with dense jirotoplasm and containing a large central nucleus, is the central cell 

 from which is finally developed the germ-cell (PI. 5, fig. 4). The upper cell now be- 

 comes divided by a wall at right angles to the longer axis of the prothallium into an an- 

 terior and posterior cell ; this wall is sometimes vertical, but more commonly inclined 

 backward so that even from the first, the anterior part of the neck is larger than the 

 posterior (PI. 4, fig. 31, PI. 5, fig. 2). A wall is next formed at right angles to the 

 first so as to divide the cell into four parts (PI. 5, fig. 1). These are the beginnings of 

 the four rows of cells composing the neck of the mature archegonium. The pi-otoplasra 

 of these cells is nearly destitute of granules, so that they are nearly colorless, and this 

 characteristic is maintained. Of the four cells into which the primary neck-cell is first 

 divided the two lying in front grow moi-e rapidly than the others, so that the neck very 

 early shows a decided curvature backward, the amount of curvature depending largely 

 upon the degree of inclination of the first formed wall. Each of the four cells next di- 

 vides by a transverse wall and later by a series of walls parallel to the first (PI. 5, fig. 2). 

 The first of these walls seems to appear somewhat earlier in the two anterior cells, the 

 two rows of neck-cells developed from these containing, usually, about two more cells 

 than are fountl in the other two, there being ordinarily six or seven in the anterior, four 

 or five in the posterior rows. 



Each neck-cell in the mature archegonium has a distinct nucleus lying close to the 

 wall, the latter being lined with a thin laj^er of very finely granular jjrotoplasm in which 

 are embedded a few larger granules, some of which show a faint greenish color. Witli 

 this exception the contents of the cells are perfectly colorless (I'l. 5, figs. G, 7). 



