59 
The Pairing of the Chromosomes. 
From such evidence the possibility emerges that the homologous 
chromosomes may in some cases remain apart until meiosis has 
begun and may in others become associated in pairs at a very early 
stage of the life-history,—either at, or soon after fertilization. It 
is evidence in favour of the occurrence of an early association that 
certain examples have been lately reported of the arrangement of 
chromosomes in pairs during the prophase or on the spindle of the 
somatic divisions. This has been perhaps described most clearly 
by Muller’ in the case of Yucca. 
Indeed, though the presence of a double somatic spireme can 
by no means be taken as implying the approximation of paternal 
and maternal elements, it is not difficult to suppose that the stage 
of the life-history at which such association takes place may vary 
in different species. 
We are aware of similar wide differences in the behaviour of 
the sexual nuclei. As a rule these fuse as soon as they come 
together, forming a single structure in which the two constituents 
cannot be discriminated; in other cases, as in several of the 
Pinaceae, 2 complete fusion is delayed until the first division of the 
oospore, or, as in certain animal embryos, till a considerably later 
stage; or, finally, in the Uredineae, the nuclei which become 
associated in the aecidium remain morphologically distinct throughout 
a long series of cell divisions and only unite at last on the eve of 
meiosis. 
We are driven here to suppose that at some stage or other of 
the life-history an attraction arises between the gametic nuclei (or 
their equivalents) which is sufficiently powerful to bring them 
together within the same membrane. No doubt in primitive forms 
fusion of the nuclei immediately followed their association and was 
in its turn succeeded by reduction. The subsequent postponement 
of the reduction stage would leave a longer or shorter period during 
which each cell would contain the elements of two nuclei; these 
would generally be united within one membrane, though throughout 
the sporophyte of the Uredineae and in the early stages of some 
other forms they remain distinct. 
With an increasing knowledge of the behaviour of the chromo¬ 
somes it appears as though some gradations subsequent to fusion 
1 “ Uebcr Karyokinetischc Bilder in den Wurzelspitzen von Yucca." 
Jahrb. wiss. Bot., 1909. 
2 V. H. Blackman, “On the Cytological Features of Fertilization 
and related Phenomena in Piuus sylvcstris." Phil. Trans. 
Roy. Soc., 1898. Noren, “ Ueber Befruchtung bei Juniperus 
communis." Arkiv. Bot. Svensk. Vetensk. Akad., 1904, etc. 
