Davis . — The Origin of the Archegonium. 491 
the evolutionary stages required by this theory of the origin of 
the archegonium and antheridium. And at the end (Fig. 22) 
we have arranged certain groups of Algae in relation to one 
another and to a problematical region of extinct forms which 
are supposed to have existed and been directly responsible 
for the Bryophytes. 
The families Ulothricaceae, Chaetophoraceae, and Coleo- 
Fig. 21. — Diagrams illustrating the possible evolution of the archegonium and 
antheridium from the plurilocular sporangium, a. Plurilocular sporangia, with 
large and small gametes discharged from the apex, after the habit found in certain 
Phaeophyceae (e. g. Chilionema Nathaliae l , Ectocarpus virescens 2 , &c.). b. 
Plurilocular gametangia of a hypothetical algal type which has adopted terrestrial 
habits. The outer layer of gamete mother-cells has become sterilized as a pro- 
tective capsule enclosing the fertile tissue. The gametes are differentiated in sex but 
both are still motile, c. Plurilocular gametangia of somewhat higher hypothetical 
forms at the level of heterogamy. Sterilization has proceeded so far in the female 
gametangium that only a few gametes are matured at the base of the organ, and 
these are eggs. d. Simple types of archegonium and antheridium. The female 
gametes are reduced to one, while the number of male gametes is greatly increased, 
and these cells are smaller and more highly specialized than in the earlier 
conditions. 
chaetaceae of the Confervales are closely related to one 
another and seem to constitute a line of ascent. Among 
the lower representatives of these families are several forms 
(Schizomeris, Draparnaldia , &c.) whose zoospores are pro- 
1 Sauvageau, ‘ Sur quelques Myrionemac^es.’ Ann. d. Sci.Nat.,8 e ser., v, 103,1898. 
2 Id., ‘ Sur r Ectocarpus virescens , Thuret.’ Jour. d. Bot., x, 17, 1896. 
