484 Davis . — The Origin of the Archegonium. 
gives us a structure that may throw some light on our 
problem. This structure is the plurilocular sporangium, and 
it is found in a number of the lower groups of the Phaeo- 
phyceae. The lower Phaeophyceae are represented by a 
number of families whose vegetative structure is diverse, but 
which agree in having one or both of two types of re- 
productive organs. There is the unilocular sporangium, a 
sporocyst, whose products are asexual zoospores ; and there is 
the plurilocular sporangium whose products, likewise biciliate 
zoospores, are known to be sexual in many forms. 
The sexuality of the plurilocular sporangium, while well 
established in certain types, is nevertheless far from universal 
in the group. It is well known from studies among the 
Ectocarpaceae that the zoospores from plurilocular sporangia 
may germinate without conjugation, and the external factors 
that determine sexuality are in part understood. So in 
considering the plurilocular sporangium we are dealing with 
a very simple and primitive type of sexual organ. The 
plurilocular sporangium is plainly a modified filament or 
branch. It consists at first of a row of cells, but shortly 
most or all of these begin to divide by walls in three planes, 
until the space originally occupied by one large cell is divided 
up into very many small cubical compartments (loculi), which 
give the structure a curious checkerboard-like appearance. 
A biciliate zoospore or gamete is formed in each of these 
compartments. Certain cells generally remain undivided at 
the base of the branch, constituting a stalk ; and sometimes 
the tip remains sterile as a hair-like continuation of the axis. 
Such is the structure of the simplest plurilocular sporangium. 
The higher types of this organ present some important modi- 
fications. The sterile tip is transformed entirely into repro- 
ductive tissue, so that the structure has less the appearance of 
a modified branch and more that of a specialized reproductive 
organ. There is also presented in certain forms (best known 
from studies among the Ectocarpaceae) a wide range of 
variation in the size and number of the compartments. Some 
of the sporangia have rather large compartments, and their 
