246 Campbell . — On the development 
The prothallium (Plate XIII, Fig. 28) now consists of four cells, 
the discoid basal cell, h } the two peripheral cells, p p\ and the 
central cell, c. As is usually the case, the mother-cell of the 
archegonium is distinguished from the other cells not only by 
its position, but also by its more densely granular protoplasm. 
The nucleus is also larger. It occupies the centre of the cell, 
and has a well-defined membrane. It is oval in form, and 
has chromatin-bodies of nearly round shape. A nucleolus does 
not seem to be present in most cases, though once a body 
was seen that may have been a nucleolus. 
About the time that the mother-cell of the archegonium is 
formed, the basal cell undergoes division by a vertical wall 
into two nearly equal cells. 
According to Arcangeli 1 there is an almost regular con- 
centric arrangement of the cells of the basal part of the pro- 
thallium, but numerous sections failed invariably to show 
anything approaching his figures. The first wall (Pl. XIII, 
Fig. 35, 1) generally can be distinguished even after numerous 
divisions have taken place, and the radial walls, 2, which suc- 
ceed this can also frequently be traced in the later stages, but 
the number of these secondary walls is so variable, and the 
succeeding ones so very irregular, that beyond the first three 
or four divisions it is quite impossible to distinguish any 
regular succession in the order of division. 
The order, so far as it can be traced, is as follows. After 
the first wall (PI. XIII, Fig. 35, 1) is formed, a number of 
secondary walls, 2, are formed running from the primary 
wall to the circumference but not strictly radial, and variable 
in number. The tertiary walls, 3, run from the secondary 
walls to the circumference, and like these are usually some- 
what curved. The next series of walls are tangential, but 
beyond this no regular order seems to prevail. In consequence 
of the variable number of the secondary and tertiary walls, as 
well as the subsequent differences in the arrangement of cells, 
the resulting cell-complex is extremely irregular, and differs 
widely in appearance in different individuals. The marginal 
1 1. c., Plate VIII, Fig. 4 . 
