Davis — The Origin of the Archegonium . 485 
products are well-developed zoospores, deeply coloured by 
the brown chromatophores. The compartments of other 
sporangia are much smaller, and develop minute zoospores 
that are sometimes almost colourless. These conditions 
are shown in Fig. 21, a. Between these extremes there is 
frequently a sporangium with medium-sized cells whose pro- 
ducts strike an average between the large and small zoo- 
spores. 
The large and medium-sized zoospores may germinate 
directly if the conditions are not favourable for sexuality. 
The small zoospores have been known to settle down and 
germinate, but the results were feeble sporelings that could 
not live long. When sexuality is present the conjugation is 
usually between the large and small zoospores. The large 
gametes swim much more slowly than the small, and in 
certain forms ( Ectocarpus siliculosus y E. secnndus, and Cut- 
leria) settle down as motionless cells which are fertilized by 
the small motile gametes. The latter have evidently reached 
a stage in their differentiation very similar to sperms both as 
to structure and behaviour. 
It is evident that the plurilocular sporangium was not 
established from the first as a gametangium, because there is 
such a large amount of parthenogenetic development among 
its products. The structure at the outset was probably 
asexual. But it is evident that with sex once established the 
evolutionary direction is along lines exactly parallel with 
the history, so well understood, for several divergent and 
independent lines of Algae. Briefly stated, we observe the 
tendency to differentiate the gametes as to size resulting in 
small male and large sluggish female cells. The latter even 
behave like eggs in certain forms at the time of fertilization, 
when they are motionless. Although the type of sexual 
reproduction is isogamous, because the gametes are identical 
in form at the time of their discharge from the gametangia, 
still the conditions at the moment of fertilization are physio- 
logically those of heterogamy. 
Another peculiarity of the plurilocular sporangium should 
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