226 CHARLES W. METZ 



In my experience good preparations have been obtained only 

 when the gonads or small bits of tissue were dissected out and 

 fixed separately- — never when the whole insect, or a consider- 

 able part of it was fixed intact. The latter method, which is 

 apparently the one used by Taylor and by Lomen, produces a 

 clumping or running together of the chromosomes, which is 

 exactly the kind of beha\dor that would cause pairs to give the 

 appearance of single chromosomes. Any tendency toward 

 fusion is especially apt to exhibit itself in the anaphases, and 

 hence it is to be expected that such figures as those obtained 

 by Taylor and by Lomen would result whenever the fixation 

 was defective. I have frequently obtained such a result when 

 the fixation was poor, especially after Bouin's, Gilson-Carnoy's 

 or alcohol-acetic fixatives. 



DETAILS OF CHROMOSOME BEHAVIOR DURING ONE CELL- 

 GENERATION 



The mutual relationship of homologous chromosomes dur- 

 ing the various stages of cell division has been carefully studied 

 in both somatic and early germinal tissues of several species, 

 and it is beUeved that the main facts regarding this relationship 

 are now evident. In brief they are these: In metaphase, either 

 in somatic cells, oogonia or spermatogonia, the chromosomes 

 lie in a flat equatorial plate, the two members of each pair, 

 with occasional exceptions, being arranged side by side as de- 

 scribed above (figs. 1, 2, 3, 17, 19, 20, etc.) Each of these chromo- 

 somes splits longitudinally, and during anaphase sends a daugh- 

 ter half to either pole, still associated with its mate from the 

 other member of the pair. Figures have already been given 

 (7, 8, 9, 16, 28, 40) showing the chromosomes in the act of split- 

 ting, or the daughter halves in the act of separating from one 

 another, also figures (12, 30, 31, 95, etc.) showing later stages 

 in which the halves have become well separated and are going 

 toward their respective poles. Retention of the paired associa- 

 tion during anaphase is evident in all, except those in which one 

 or two pairs have' been disarranged. In the telophase, the 

 chromosomes become closely massed and rapidly lose their 



