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 “2n” 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 (Zc. 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 ; muclear 
reduction, or fuston, 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 Sgrrogyra 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 gametogenests and 
sexual fusion. The order of events would then be (a) somatic division 
with “2n” chromosomes: (4) chromosome-reduction: (¢) gametogenesis 
and sexual fusion. This is the case generally for animals: in plants the 
best demonstration has been in Fucus: it is also seen probably in AZopalodia ; 
but it probably occurs also in many of those Thallophytes which have 
no obligatory succession of phases, and especially in Achiya, 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: (4) 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 whith a somatic phase of some extent intervenes between sexual 
Jusion 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 Site. d. Ges. a. Wiss. in Prag, 1874, p. 30. 
