85 



The Behavior of the Chromosomes in Pinus and Thuya. 



I. M. Lewis. 



A more uiiiforiii iutei-pretation prevails iimou;,' botiiuists today cou- 

 cerniDg the reduction of chromosomes in tlie spore mother cells of the 

 higher seed plants tlian at any time in the history of this most perplex- 

 ing of all questions. The view once prevalent that the reduction is brought 

 about by two longitudinal fissions of the chromatin has been almost en- 

 tirely abandoned, and there is a growing belief which is now almost uni- 

 versal that a true nunierical reduction takes place as proposed by Weis- 

 maun several years ago, although this occurs in the first instead of the 

 second mitosis. 



The question of the occurrence of a true reducing division may be 

 talcen, therefore, as quite definitely settled. The investigations which have 

 brought about this condition have, however, raised new questions which 

 are almost as difficult. Chief among these questions may be mentioned : 

 the individuality of chromosomes, the origin of chromosomes from the 

 spirem and the manner in which the characters are distributed to the germ 

 cells. Concerning these questions there are various conflicting views. It 

 is quite generally accepted among cytologists, that the maternal and pa- 

 ternal chromatin i-emain in a state of comi)lete segregation throughout the 

 growth phase of the organism and that they become more or less completely 

 united at the time of synapsis. The behavior of c-hromosomes in hybrid 

 forms has had a great influence in bringing about this conception. 



It is concerning the origin of the chromosomes from the resting neu- 

 cleus and their union in synapsis that the widest difference of opinion is 

 held. There is one group of investigators among whom Strasburger, Guig- 

 nard, Allen, Overton and others are prominent who maintain that the 

 chromosomes of each ancestry become arranged into a complete spirem pre- 

 vious to synapsis, that these two spirems then approach each other and 

 apparently fuse side by side into one during the synaptic phase. Follow- 

 ing synapsis the spirems again separate somewhat and cross segmentation 

 takes place. The somatic chromosomes which have been so united side 

 by side become variously oriented toward each other and thus give rise 

 to the heterotype bivalents typical of this mitosis. 



