DEVELOPMENT OF THE OSTRICH FERN. 27 



]:)arallel to the surface of the prothallinm, so that the four-celled embryo, seen from 

 above, appears to have only two cells (PI. 5, figs. 11, 17). Each of the four cells is 

 next divided by a wall (the median wall) parallel to the long axis of the prothallinm, so 

 that the embryo now consists of eight cells arranged like the octants of a sphere. 



Up to this point, as far as is known, the majority of ferns seem to agree, except as to 

 the relative time of the formation of the transverse and median walls, which appear in 

 different order in different species. As to the fuither history of the eight primary cells 

 of the embryo, there is considerable disagreement. After repeated examination of a 

 large number of specimens, the following conclusions have been reached with reference 

 to the fei'n nnder consideration. 



If we examine again the eight-celled embryo, it is easily seen that each one of the cells 

 is nearly tetrahedi-al in fcjrm ; that is, the walls that bound the faces that are in contact 

 are approximately plane triangles, the outer wall a spherical triangle. Each of the eight 

 cells thus has tlieform characteristic of the ajjical cell of the mature stem and root. For 

 a short time these all grow in essentially the same way as the a])ical cells of these or- 

 gans, but this growth is checked in all but those two that finally assume the function 

 of these two cells. 



Of the four cells first formed, the tAvo lying in front of the basal wall give rise to the 

 first leaf and the stem; those of the hypobasal half of the first root and foot (PI. 5, 

 fig. 11). Of the epibasal portion, the one next to the neck of the archegonium forms 

 the first leaf, the othei- the stem; of the hypobasal half, the lower (next the neck of the 

 archegonium) the first root, the upper the foot. 



The first divisions are essentially the same in all the octants, though there is slight va- 

 riation in some cases. The first wall (PI. 5, fig. 14, 1) is usually parallel to the basal 

 wall, and is soon followed by a second, parallel to the transverse wall; sometimes this 

 order is reversed, but never, so far as could be made out, was the first wall in the octant 

 parallel to the median wall. Befoi-e any further segments are cut off from the primary 

 cells (octants), these first segments undergo further divisions. Each one is divided into 

 two cells by a tangential wall, an inner and an outer cell (fig. lo). Each octant now 

 consists of five cells, two inner and three outer, one of the latter being the primarj^ cell 

 which still retains its tetrahedral form. After a segment has divided into an inner and 

 an outer cell, the latter becomes divided into two by a radial wall, but beyond this the 

 succession of walls differs. Of these eight tetrahedral cells, four, one in each quadi-ant 

 of the embryo, soon lose their primitive form : indeed this sometimes occurs even before 

 the second set of segments is cut off" from the primary cells. This is brought about as 

 follows : In one of the primar}- cells of each quadrant, after the first or second divisions 

 have taken place, instead of new segments being cut off in the same way, a tangential 

 wall is formed, dividing the primary cell into an inner and an outer one. The outer one 

 is divided by radial walls ; and the inner, dividing by walls directed in various directions, 

 soon becomes indistinguishable. There are thus left remaining four of the primary cells, 

 as the apical cells of each quadrant. Of these, those of the stem and root remain un- 

 changed; that of the leaf undergoes some further changes, to be presently' described; 

 while that of the foot, after one or two more sets of segments have been cut off, is di- 

 vided by a tangential wall into an inner and outer cell, in the same way described in the 

 other pi'imary cells, and thus ceases to act as an apical cell. 



