yO THE ALGAE 



develop one day later than the oogonia, thus ensuring cross- 

 fertilization. 



The oogonia are enlarged spherical or ellipsoidal cells arising by 

 one division in which the upper segment forms the oogonium and 

 the lower a support cell, or else the latter subsequently divides to 

 give antheridia. In some species the lower cell may also become an 

 oogonium so that one can find a series of oogonia on one filament. 

 Each oogoniimi contains one ovum with a colourless receptive spot 

 situated opposite to the opening in the oogoniimi wall from which 

 a small quantity of mucilage is extruded. The opening is either a 

 very small pore, formed by gelatinization of a tiny papilla, or else 

 a slit, but in either case there is an internal membrane forming a 

 sort of conduit to the ovum. After fertiUzation the oospore often 

 becomes reddish in colour and develops a thick membrane which is 

 usually composed of three layers. The markings on the outer mem- 

 brane are important taxonomically. At germination the protoplast 

 divides into four segments, which may each develop flagella and 

 escape as zoospores, or else they function as aplanospores that later 

 give rise to zoospores. Meiosis takes place at the germination of the 

 zygote so that the adult filaments are haploid. In one species it has 

 been definitely established that two of the zygote segments ulti- 

 mately develop into male plants and two into female plants. Zygote 

 germination vdthout meiosis is not uncommon, in which case it 

 gives rise to what are presumably large diploid swarmers, and these 

 develop into abnormally large threads that are always female. 

 Oogonia appear on these diploid filaments and can be fertihzed, 

 but the fate of the zygote is unknown. 



It remains to discuss the possible origin of the androspores, and 

 there are two hypotheses that may be considered : 



(a) The androspore is equivalent to a second and smaller type 

 of asexual zoospore, such as the microzoospores found in 

 some species of UlothriXy but in the Oedogoniales they can 

 no longer give rise to normal filaments. On this view the nan- 

 nandrous forms are the more primitive, the macrandrous 

 having been derived by the androsporangiimi acquiring the 

 capacity to produce antheridia immediately and hence never 

 appearing. 

 {h) The androspore is equivalent to a prematurely liberated 

 antheridial mother cell which subsequently undergoes 

 further development. On this view the macrandrous species 



