288 
Lang . — The Prothallus 
19. In the latter, however, the series of central cells is 
complete. In the mature archegonium. the neck usually 
projects considerably from the surface (Fig. 20), though only 
the upper portion of its wall is a single layer of cells thick. 
The lower portion appears to be produced by the surrounding 
cells of the generative layer keeping pace with the enlargement 
of the archegonium at maturity, and is not derived from the 
outermost cell of the young archegonium (Fig. 17) ; this 
region, which is as a rule more than one layer of cells thick, is 
thus not strictly comparable to the archegonium-neck in other 
Vascular Cryptogams. In the archegonium represented in 
Fig. 2i, this division and growth of the cells of the generative 
layer has on one side not been confined to the cell immediately 
adjoining the archegonium. The venter of the archegonium 
and the lower portion of the canal are surrounded by a layer 
of small cells cut off from those of the generative zone. These 
cells have dense protoplasmic contents, and their nuclei are 
frequently to be found against the wall which bounds the canal 
of the archegonium (Figs. 20 and 21). Within the venter is 
the large ovum, and the canal is occupied by six to eight 
canal-cells, seven being the number most frequently observed 
(Figs. 20 and 21) ; the lowest of these was not distinguished 
with any certainty as a ventral canal-cell, since the material 
was insufficient to allow of the succession of divisions in the 
two inner cells of the young archegonium being traced. 
Transverse sections of the young archegonium-neck show that 
it is typically composed of four rows of cells (Fig. 22), though 
subsequent divisions in one or more of these may increase 
the number. In sections a little lower down, the wall of the 
neck-canal is found to be more than one layer thick, while 
still lower down the ovum is found embedded in the prothallus. 
These relations will be sufficiently apparent from the longi- 
tudinal section (Fig. 20). The details of the opening of the 
archegonium-neck were not observed. 
