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 homceotypic 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. 



Farmer and Moore 

 Farmer and Digby 

 Farmer and Shove 

 Mottier 



Gregory 



Lewis 



Schaffner 



Digby 



Fraser 



Lawson 



McAvoy 



Beer 



Woolery 



Nothnagel 



Farmer and Moore 

 Montgomery 



Moore and Embleton 



Griggs 



Zweiger 



H. S. Davis 



Nakahara 



PLANTS 



1903, '05 Lilium, Osmunda, Psilotum, Aneura 



1910 GaUonia 



1905 Tradescantia 



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

 Tradescantia, Staphylea 



1904 Ferns 



1908 Finns, Thuja 



1906, '09 Agave 



1910, '12, '14, '19 GaUonia, Primula, Crepis, Osmunda 



1914 Vicia 

 1912 Smilacina 

 1912 Fuchsia 



1912, '13 Equisetum, Crepis, Tragopogon 



1915 Smilacina 



1916 Allium 



ANIMALS 



1905 Periplanela, Elasmobranchs 

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



'10 



1906 Amphibia 



1906 Ascaris 



1907 Forficula 



1908 Insects 

 1920 Perla 



