134 Johnson. — On the Development of the 
rise to part, if not all of the ventral portion of the gelatinous 
ring. The inner portion however, in the basiscopic half of 
the segments at least, remains of a single cell in thickness in 
each, even at maturity. 
We come now to the most important division of the soral 
segment, the basiscopic ultimate marginal cell (Figs. 34-38), 
from which are derived all the sporangia of the sorus. This 
is the ‘ Sorusmutterzelle ’ of Biisgen, but this name seems 
inappropriate as there is no single mother-cell of the sporangia 
of the sorus alone, nor of the whole sorus including the 
indusium after the single marginal cell of the third grade. 
No dermatogen-wall is formed in these marginal cells, and 
the sori being derived thus, like the young sporocarp itself, 
from a cell capable of forming at least two of the meristem- 
layers, are not of strictly epidermal origin. As the young 
sporocarp increases in size, we find that soon after the forma- 
tion of section VI the basiscopic marginal cell elongates in 
the direction of the length of the organ, and divides by 
a transverse anticline into halves, of which the acroscopic 
one soon comes to be the larger (Figs. 34-38). Then each 
of these divides by another anticline (Fig. 38), forming thus 
four cells, of which the basiscopic one of the acroscopic pair 
soon becomes the largest ( p . ma-sp. m. c ., Fig. 38), while its 
sister-cell on the acroscopic side splits by still another anti- 
cline. We have formed thus a series of five cells, of which 
the middle and larger one (p. ma-sp. m. c., Figs. 34, 38) is 
the primary macrosporangium mother-cell giving rise to all 
the macrosporangia of the sorus. The adjacent cells on 
either side of the latter (p. mi-sp ., m. c., Figs. 34, 35, 38) are 
the primary microsporangium mother-cells, while the outer 
cells of the five (i. ind ., Figs. 34, 38) give rise to the inner 
layer of the indusium on each side. The outer layer of the 
indusium on each side is formed by the splitting in two of 
the acroscopic marginal cell by a transverse anticline ( 0 . ind., 
Figs. 35-38), one half helping to form the indusium of the 
sorus of its own segment, and the other of the sorus of the next 
younger segment. 
