246 CHARLES W. METZ 



to the development of this theory probably no single concep- 

 tion has been of. more value than that which postulates a quali- 

 tative differentiation among the chromosomes (Boveri '01), 

 and an individual homology between respective members of 

 the two gametic groups (Montgomery '01). The growth of 

 this conception is of particular interest in the present connec- 

 tion.^ It was based, of course, upon the foundation laid by 

 Van Beneden's 'law' ('83) of the equivalence of maternal and 

 paternal chromosome groups, and upon the principles of chromo- 

 somal individuality and continuity developed by Rabl ('85), 

 Boveri ('87, '88, '91), Herla ('93), Zoja, Van Beneden and others, 

 but not until 1901 did it assume its present features. From 

 Montgomery first came the idea that each chromosome in the 

 spermatozoon has an equivalent mate in the egg, that fertiliza- 

 tion brings the two together in one cell, and that maturation 

 segregates them again into different cells — the gametes,^ These 

 conclusions were based upon a study of several Hemiptera 

 (Protenor, Peliopelta, Zaitha), in which certain pairs of sperma- 

 togonial chromosomes, distinguished by size and shape, appar- 

 ently became associated in synapsis and underwent segregation 

 in the reduction division. The almost simultaneous and even 

 more far-reaching observations of Boveri ('01) were from his 

 well known experiments on dispermic sea-urchin eggs, in which 

 he demonstrated a qualitative difTerence between the respective 

 chromosomes in their effect upon development. 



Further attention may be confined to features relating to 

 chromosome pairing. Xhe first of these is the discovery by 

 Montgomery in 1904 and 1905 of a paired association of corre- 

 sponding chromosomes in cells other than those involved in the 

 maturation process. These observations were made upon 

 Plethodon, and upon the Orthopteran, Syrbula, in the latter of 

 which he found twelve of the twenty chromosomes to possess 

 size differences enabling him to assort them into six groups of 



' For a comprehensive review see Wilson '05, '14, Conklin '14, East '15, and 

 Morgan, Sturtevant, Muller, Bridges '15. 



' This conclusion was forecasted perhaps by Henking in 1891, and by Mont- 

 gomery in 1900, but was first given definite expression by Montgomerj- in 1901. 



