.HCIDIOSPORES 5 



in the dividing wall which afterwai'ds 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 1 . 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 ai - e 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 ivork- 

 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) ami Spirogyra (Trondle, 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 Algae (Griffithsia, Dictyota) the intercalation takes place on an 

 extensive scale, both between (2) and (3) and between (3) and (1). 



