ZCIDIOSPORES 5 
in the dividing wall which afterwards enlarges until at length 
hardly any trace of the wall is left. The two cytoplasms thus 
form one mass, but the nuclei arrange themselves more or less 
side by side without fusing or in most cases even touching. 
This process must be regarded as an act of fertilisation or 
rather as a substitute for such an act. The double cell is 
called a fusion-cell. 
The two adjacent nuclei are said to be “ paired,” and to- 
gether constitute a synkaryon or dikaryon. Occasionally, in 
some species of Puccinia and other genera, these’ fusion-cells 
contain three nuclei, probably by a double fusion. Such cells 
produce trinucleate spores, but the fate of these is not known. 
The paired nuclei of the fusion-cell then divide side by side 
and simultaneously—a process called conjugate division—and a 
wall is formed between the two pairs. The wall is formed in 
such a way that the two nuclei in the same cell are not sister- 
nuclei (see Figs. 19, 20). 
The fact that in conjugate division the paired nuclei divide 
so that the two are usually in the same stage of mitosis at the 
1 The stages connected with the sexual process in general are three in 
number—(1) the association of two (almost always non-sister) nuclei in the 
same cell, (2) the fusion of the two nuclei, preparatory to (3) the reduction in 
the number of chromosomes, or at any rate in the amount of chromatin, to the 
previous ordinary vegetative condition. These three stages may or may not 
follow closely upon one another. The first and second, or the second and 
third, or the third and first, may-be separated by an intervening series of cell- 
divisions. Which is to be regarded as the actual fertilisation? In all proba- 
bility the first. The nucleus is the director, the cytoplasmic mass is the work- 
people of the cell-factory. The presence of the two directors is the essential 
fact. The fusion is of less importance; it is often delayed for a long period, 
and in certain cases it is known that, even when fusion has taken place, the 
chromosomes still retain their individuality for a considerable time. There 
is reason to believe that on the first introduction of fertilisation these three 
stages followed immediately (i.e. without intervening cell-divisions) after one 
another, the series of vegetative divisions being intercalated between (3) and (1), 
as in Coleochaete (Allen, 1905) and Spirogyra (Tréndle, 1907; Karsten, 1909). 
In the majority of the higher plants, stages (1) and (2) occur without much or 
any interval, but a long series of divisions (the sporophyte) is intercalated 
between (2) and (3). In most of the Uredinales, the chief series of vegetative 
divisions is intercalated between (1) and (2), and (3) follows immediately after 
(2). In certain Alge (Griffithsia, Dictyota) the intercalation takes place on an 
extensive scale, both between (2) ‘and (3) and between (3) and (1). 
