THE REDUCTION OF THE CHROMOSOMES 239 



thrown into loops (" second contraction"). These loops then break 

 apart from one another through a segmentation of the spireme; each of 

 them is composed of two split chromosomes arranged end-to-end. Chro- 

 mosome conjugation has thus occurred telosynaptically (metasyndetic- 

 ally] either while the spireme was being formed or when the daughter 

 spiremes were formed in the preceding telophase. The two members of 

 each pair are brought around to a side-by-side position by the looping 

 at the second contraction, usually but not always remaining closely 

 connected at the original point of conjugation. The resulting bivalent 

 chromosomes, with their split obscured, become much shortened and 

 thickened (diakinesis) and take up their positions on the first maturation 

 spindle. In case the original split, instead of being wholly obscured, is 

 visible at this time or earlier, chromosome tetrads are evident. In the 

 heterotypic anaphase the bivalents are separated into their component 

 univalents, bringing about reduction. During the anaphase the uni- 

 valents often widen out along the line of fission which had been tempo- 

 rarily obscured, giving them the form of simple or double Vs as described 

 for Scheme A. They remain through interkinesis in the double condi- 

 tion, and in the homoeotypic mitosis separate along this line of fission. 



The following is a list of the principal works in which this theory of 

 eduction has been advocated. 



PLANTS 



Farmer and Moore 1903, '05 Lilium, Osmunda, Psilotum, Aneura 



Farmer and Digby 1910 Gallonia 



Farmer and Shove 1905 Tradescantia 



Mottier 1907, '09, '14 Lilium, Acer, Allium, Podophyllum, 



Tradescantia, Staphylea 



Gregory 1904 Ferns 



Lewis 1908 Finns, Thuja 



Schaffner 1906, '09 Agave 



Digby 1910, '12, '14, '19 Galtonia, Primula, Crepis, Osmunda 



Fraser 1914 Vicia 



Lawson 1912 Smilacina 



McAvoy 1912 Fuchsia 



Beer 1912, '13 Equisetum, Crepis, Tragopogoh 



Woolery 1915 Smilacina 



Nothnagel 1916 Allium 



ANIMALS 



Farmer and Moore 1905 Periplanela, Elasmobranchs 



Montgomery 1903, '04, '05, '06, Hemiptera, Amphibia 



'10 



Moore and Embleton 1906 Amphibia 



Griggs 1906 A scans 



Zweiger 1907 Forficula 



H. S. Davis 1908 Insects 



Nakahara 1920 Perla 



