Davis . — The Origin of the Archegonium. 487 
cells, and, since the cells develop zoospores simultaneously, 
strikingly resembles a plurilocular sporangium. The condi- 
tions among the lower Ectocarpaceae, especially in Pylaiella , 
are no more complex than in Schizomeris ; so we have in the 
Chlorophyceae structures that might readily be the fore- 
runners of well-differentiated plurilocular sporangia. 
A plurilocular sporangium is subject to two sets of factors 
that may influence its form and structure, together with the 
character of the sexual cells. There are, first, the general laws 
underlying all sexual evolution in its advance from isogamy 
to heterogamy. And in addition to these developments there 
are the changes possible in any multicellular organ, because it 
is a cell-complex, and may be differentiated into tissues. 
A gametocyst (single cell) is by its simplicity barred from the 
complications possible to a gametangium. 
The differentiation of the gametes into eggs and sperms 
is readily understood along the line that we have already 
suggested, which is a well-known path of development, and 
has been travelled by many groups of Algae. We know that 
the gametes vary in size, and that the larger female elements 
are sluggish and tend to settle down before fertilization as 
quiescent cells, to which the male gametes are attracted. 
Should the sluggishness of the female gametes be intensified 
some of them might not be able to leave the gametangium, 
but would remain there as eggs, retained on the parent plant, 
to which the male gametes must make their way. It is 
not at all uncommon for zoospores of various Algae to be 
mechanically held within a parent sporangium and, unable 
to escape, to germinate there. Such habits on the part of 
female gametes of plurilocular sporangia would finally result 
in heterogamy, with the retention of the eggs within the 
parent gametangium. 
What are the possibilities of modifications in form and 
structure of the plurilocular sporangia themselves ? These 
would depend on two important factors, first the sterilization 
of portions of the structure, and second the differentiation of 
regions of exit or entrance for the gametes. Modifications 
