MEIOSIS 283 



the rusts, meiosis ordinarily occurs as the fusion nucleus in the teliospore 

 divides to form the four nuclei of the sporidia.^^ 



Meiosis in bryophytes^^ and vascular plants takes place in the divi- 

 sions produci-ng the nuclei of the spore quartets. 



Conclusion. — From the foregoing account it may be concluded that 

 meiosis is a process which probably occurs in some form in all nucleated 

 organisms reproducing normally by sexual means. Haplosis in some 

 form is obviously a necessary consequence of syngamic nuclear fusion. 

 The view long current that synapsis is itself a sexual process, the "cul- 

 mination of fertilization," is, however, rendered less easy of acceptance 

 by what has been ascertained regarding synapsis (1) in polyploids and 

 hybrids, where the synaptic mates are often from the same gamete, (2) in 

 certain parthenogenetic organisms, and (3) between different parts of the 

 same chromosome, exceptional as this may be. Theorizing on this 

 subject will be more profitable when more has been learned about the 

 forces actually concerned in the "attractions" of cells, nuclei, chromo- 

 somes, and their constituent elements in the life cycles of different 

 organisms. 



The cell in which meiosis is initiated (the meiocyte) may be a sporo- 

 cyte, a gametocyte, a zygote, an ascus, a basidium, or some other cell; 

 but whatever the relative position of syngamy and meiosis in the life 

 cycle, these two cytological crises are events of the highest significance 

 with respect to the reproduction of the organism. The following chap- 

 ters will deal with phenomena of heredity exhibited chiefly in successive 

 generations of plants and animals reproducing sexually; hence, if the 

 essential features of chromosome behavior in the two cytological crises 

 are not borne clearly in mind, these chapters will not be intelligible. 



62 W. H. Blackmail (19046), Dietel (1911), Fitzpatrick (19186), Colley (1918), and 

 others. See Arthur (1929), Gaumann-C. Dodge (1928), B. O. Dodge (1929a), and 

 Jackson (1931) for accounts of the various types of life cycle in rusts. 



63 Farmer (1894, 1895) and A. C. Moore (1903, 1905) on Pallavicinia; B. M. 

 Davis (1899, 1901) on Anthoceros and Pellia; Walker (1913) and Vandendries (1913) on 

 Polytrichum; Melin (1915) on Sphagnum; C. E. Allen (19176, 1919) and Schacke (1919) 

 on Sphorrocarpos; Florin (1918a) on Chiloscyphus; Blair (1926) on Reboulia; Heitz 

 (1928o6) on Pellia; Lorbeer (1924, 1927) on Anthoceros, Sphcerocarpos, and Pellia. 

 The cytology of bryophytes is reviewed by Motte (1929). 



