637 



to which it would be inapplicable. On our part there was never the 

 slightest idea that such lowly plants would furnish good examples of 

 the origin of vegetative cells from sterilized gametes. As the pre- 

 ceding sentences (cited above) prove, our contention only related to 

 those plants in which sporophyte and gametophyte were differentiated 

 entities, i. e. to Metaphyta as corresponding to Metazoa. From our 

 standpoint the cases, such asCladothrix, Ulothrix, and S p h a e - 

 roplea, are all equivalent to colonial Protozoa, in which in each 

 and every cell there exists the potentiality of forming gametes. 



It is by no means clear from Prof. Mac Millan's statements 

 that he maintains the exact converse of our position. We hold that 

 vegetative or somatic cells, except in so far as some of them may 

 have retained their potentiality of becoming or giving origin to re- 

 productive cells (gametes or spores), must be looked upon in both 

 kingdoms as sterilized. We believe such sterilization, whether com- 

 plete or only functional with reproductive powers in abeyance, to be 

 an essential preliminary to the building up of a complex organism 

 and the formation of tissues. 



Prof. Mac Millan states that "it is certainly an inversion of the 

 facts to maintain that in gametophytes vegetative cells are always 

 sterilized gametes." 



In expressing this opinion Prof. Mac Millan is, we imagine, 

 taking up a position precisely comparable to that until recently main- 

 tained by botanists regarding spore-formation. The sporophylls of 

 higher plants were for a century or more regarded as modified leaves, 

 until Bower ^) showed that "in some cases at least, foliage leaves 

 probably have been the result of sterilization of sporophylls." 



The view we hold is, at any rate, a good working hypothesis, 

 and will, we anticipate, turn out to be the correct one. What sort 

 of a working hypothesis is that of the origin of gametes from vege- 

 tative cells? What facts does it explain, and how does it stand with 

 reference to more than one result of recent research? It is moderately 

 easy to gain some idea of the evolution of a gametophyte from a uni- 

 cellular organism, or from a row of such, on the supposition of the 

 evolution of the gametophyte going hand in hand with the sterili- 

 zation of some cells of the filament, or of some of the products of 

 fission of the unicellular organism. 



1) F. 0. BowEE, Studies in the Morphology of spore-producing mem- 

 bers, Phil. Trans. London, 1894, B. p. 482. 



