200 Atkinson . — A Monograph of the 
like the fru strum of a pyramid, its broad base resting against 
the central axis, its smaller end reaching to the peripheral sur- 
face of the mother-cell ; the other four cells are the primary 
cells of the two ascending and two descending generative fila- 
ments, Fig. 29 /, the central group of cells * here the lower 
one has not yet divided longitudinally into the two generative 
filaments. The ray-cell here not only elongates but divides by 
transverse fission to produce the tie-cell which connects it 
directly with the wall. The tie-cells in this subgenus are very 
much elongated, so that the generative filaments are separated 
from the wall. Very early in the development of the sexual 
shoot, from the underside of the bases of the ray-cells, in the 
subgenus Lemanea , two or three articulated filaments of long 
slender cells grow out, branch, and run spirally downward 
around the central axis (see Fig. 32 c). When the sexual 
shoot is mature in this species (Z. australis ), and some others, 
the central axis is entirely hidden by a compact mass of these 
filaments. 
The centrifugal development of a primary cell of the young 
sexual shoot is farther illustrated in the longitudinal section 
represented in Fig. 31. 
The farther development of the walls in both subgenera is 
as follows. The tie-cells by peripheral and radial fission pro- 
duce the cells of the medullary layer ; the cells of the medul- 
lary layer by a corresponding fission produce the cells of the 
intermediate layer, and these in turn produce the cells of the 
cortical layer. In the development of each of these layers of 
the wall, as the periphery of the wall is constantly increasing 
in superficial extent, and as the cells of each layer are smaller 
than those from which they arise, it will be seen that each 
cell of the medullary layer and intermediate layer produce 
respectively a group of several cells 1 . 
1 Ketel says (Anat. Untersuch. liber Lemanea , p. 25, 1886), ‘ Weiterhin 
wiederholen dann die zulezt entstehenden Aussenzellen denselben Theilungsvor- 
gang und theilen sich ebenfalls in eine innere und 2-3 “ aussere Zellen.” ’ He 
represents in Fig. 3 a side-view of the cells growing from a c Verbindungszelle.’ 
He has not given credit for a sufficient number of cells, since the branching of a 
