CHROMOSOME STUDIES ON THE DIPTERA II. 



THE PAIRED ASSOCIATION OF CHROMO^ 



SOMES IN THE DIPTERA, AND ITS 



SIGNIFICANCE 



CHARLES W. METZ 

 Station for Experimental Evolution, Cold Spring Harbor, N. Y. 



EIGHT PLATES 



CONTENTS 



Introduction 213 



Materials and methods 217 



Reality of chromosome pairing in the Diptera 221 



Details of chromosome behavior during one cell-generation 226 



Pairing in different tissues and during different stages in ontogeny 230 



Different species and families compared 231 



Discussion 245 



Summary and conclusions • 258 



Bibliography 260 



INTRODUCTION 



Attention was first called to the pairing of chromosomes in 

 the Diptera by Miss N. M. Stevens during 1907 and 1908 in 

 connection with studies upon the heterochromosomes of insects 

 (Stevens '07, '08). Although primarily concerned with the 

 heterochromosomes and maturation phenomena, Stevens never- 

 theless found the paired association of chromosomes, in the 

 nine species she studied, so conspicuous as to warrant the state- 

 ment that, ''perhaps the most interesting point in the whole 

 study is the pairing of chromosomes in cells somewhat removed 

 from the sphere of the reduction process. This was found to 

 occur in the ovarian follicle cells, the spermatogonia and some 

 embryonic cells. This is not an occasional phenomenon, but 

 one which belongs to every oogonial and spermatogonial mitosis" 



213 



