Origin of Sexuality 251 



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But the writer believes that extreme importance* attaches 

 to the mode of division, followed by spore formation, in the 

 Chamsesiphonese and allies as cited above. For this seems in 

 every respect to represent the commencing phase of endospore 

 formation, which plays so important a role in the higher or 

 nucleate algae, and is a usual preliminary to sex-cell formation. 



It seems highly probable that, though the Chamsesiphoneae 

 is the group which mainly shows endospore formation now, 

 members of the simple spherical genera may also have advanced 

 to a like state. But either from the simpler and more round- 

 celled Chamaesiphonese, or from evolved types of the Chroococ- 

 cacese, we would derive the typically unicellular but at times 

 nucleate PleurococcacecEy Protococcacece, and Tetrasporacece. A 

 study of sexual cell formation, and of evolving sex-cells in the 

 higher or nucleate algae, can therefore now be undertaken. 



As amongst the simpler unicellular Cyanophycese, so in 

 the above groups, direct division and rounding off of each 

 cell may give rise to new individuals. Or again "akinetes" 

 may result from thickening of the wall of a unicellular indi- 

 vidual, or of a cell in the multicellular one, with occasional 

 enlargement of it. The contents also seem often to undergo 

 change or reformation, and then, or only after a rest period, 

 do these germinate to form a new individual. In formation 

 of "aplanospores" the contents of a cell contract and round 

 off from the parent wall. The mass then secretes a new wall 

 round itself, and in time is set free by dissolution of the parent 

 membrane. This may undergo a longer or shorter rest period, 

 and then germinates into a new plant. 



The "pahnelloid spore" results from division and redivision 

 in successively different planes, at right angles to each other, 

 until a number of spherical cells are formed. These may 

 rest within the parent wall for a time, and then escape to pro- 

 duce a new plant, or they may instead give rise to the swarm- 

 spore soon to be studied. 



All of the above types of new cell formation seen in pleuro- 

 coccoid organisms are exactly shown, in some members of the 

 Cyanophycese as already noted, and in the former as in the 

 latter they have evidently resulted from periodic exposure 

 of organisms to definite envlronal states, as subjoined evidence 

 will indicate. 



But another and highly important asexual spore form is 

 that generally known as the endospore or swarmspore. Here 

 the contents of a vegetative cell may have become rounded 

 off, but more usually become divided and re-divided until 

 four to many smaller ones have arisen inside the parent mem- 



