No. 584] HEREDITY AND ITS MEANING 



467 



in the reduction divisions. The ten pairs separated nor- 

 mally, one of each pair going to each pole; but the ten 

 unpaired were irregularly distributed, sometimes nearly 

 all of them going to one pole, sometimes most of them be- 

 coming lost in the cytoplasm and forming small nuclei. 

 Embryos were produced in a very few cases and these 

 only through back-crossing with pollen of D. longifolia. 

 Unfortunately these embryos only developed through a 

 few cell divisions. 



These chromosome pairs have been distinguished by 

 the name homologous chromosomes. For a long time it 

 was thought that the paternal and the maternal set of 

 chromosomes separated from each other bodily at the re- 

 duction division. Xow it is believed to be only a matter 

 of chance which chromosome of a pair passes to a particu- 

 lar daughter cell. There is some etiological evidence for 

 this view, but the main argument in its favor is that this 

 behavior is all that is necessary to tit nearly all the known 

 facts of heredity, with the chromosomes playing the 

 part of the active heredity machinery as will be seen 

 shortly. This statement is true in a broad sense, but the 

 word nearly is used because there is an exception to it. 

 Chance apportionment of either member of a homologous 

 pair of chromosomes to a daughter cell accounts for all 

 facts of alternative (Mendelian) inheritance except where 

 there are breaks in the correlation between characters 

 usually inherited together. Since such breaks in corre- 

 lation are common, it is clear that there must be a period 

 when chromosome pairs have such an intimate relation 

 that material can be exchanged. Many biologists believe 

 that such a period is found during the maturation of the 

 sex cells. The particular point at which such a conjuga- 

 tion or approximation of chromosome pairs takes place is 

 called synapsis; it occurs as a part of the prophase or 

 first stage of the reduction division. Some investigators 

 have been unable to demonstrate any real chromosome 

 fusion at this time, but all agree that there is an approxi- 

 mation between the two sets, and a chance for some kind 

 of an exchange or interaction to take place. 



