THREE LEADING TYPES 75 



exists among them. According to the criterion of chromosome-difference 

 there may be recognised a haploid, pre-sexual phase, characterised by 

 having " n " chromosomes this corresponds cytologically to what has 

 been termed elsewhere the gametophyte ; and a diploid, post-sexual stage, 

 characterised by having "211" chromosomes this corresponds in this 

 respect at least to the sporophyte. The alternation of these phases 

 depends primarily upon sexuality, which doubles the chromosome-number. 

 The reduction of the chromosome-number to one half appears to be a 

 necessary consequence of it, and the process by which the original number 

 is restored is found to be commonly associated, here as elsewhere, with a 

 tetrad-division. 



As Professor V. H. Blackman has pointed out (I.e., p. 364), three nuclear 

 stages are to be observed in the sexual cycle of animals and plants : 

 nuclear association by fusion of the protoplasts which contain them ; nuclear 

 reduction, or fusion, which doubles the chromosome-number; and chromosome- 

 reduction, by which their number is halved. Of these three stages the 

 second may take place at the same time as the first, or it may be delayed 

 for a short time, as in Spirogyra or Cosmarium : or, as in the Uredineae, 

 it may be delayed until the stage corresponding to chromosome-reduction. 

 According to the relative time of these successive nuclear stages the sexual 

 cycle may vary greatly, as we see that it does in the Thallophytes ; and 

 three leading types of the cycle emerge, though they severally may graduate 

 into one another by intermediate steps : they include : 



1. Those in which reduction immediately precedes gametogenesis and 

 sexual fusion. The order of events would then be (a) somatic division 

 with " 211 " chromosomes : (/>) chromosome-reduction : (c) gametogenesis 

 and sexual fusion. This is the case generally for animals : in plants the 

 best demonstration has been in Fucits : it is also seen probably in Rhopalodia ; 

 but it probably occurs also in many of those Thallophytes which have 

 no obligatory succession of phases, and especially in Achlya, and probably 

 in the Peronosporeae. 



2. Those in which reduction immediately follows on sexual fusion. The 

 order of events in these would be (a) somatic division with "n" chromo- 

 somes : (b) gametogenesis with sexual fusion : (c) chromosome-reduction. 

 This is probably the case in Desmids and other Conjugatae, and in the 

 filamentous Chlorophyceae, including Coleochaete. 



3. Those in which a somatic phase of some extent intervenes l>etween sexual 

 fusion and reduction, and again between reduction and sexual fusion. This is 

 seen in Dictyota, probably in the simpler Ascomycetes, in Uredineae, and 

 Florideae : it is comparable with what is seen in the Archegoniate series. 



It is interesting to compare the grouping of types of alternation as 

 thus stated with the position adopted by Celakovsky in his paper which 

 was published some thirty years ago at Prag. 1 The data, both physiological 

 and cytological, are now much more precise, though still very deficient. 



1 Sifz. d. Ges. </. ty/'ss. in Prag, 1874, p. 30. 



