192 KANSAS UNIVERSITY SCIENCE BULLETIN. 



are divided in the metaphase, it is possible to trace the forma- 

 tion of the tetrad chromosomes and to be sure of the relation 

 existing between the longitudinal and cross planes of separa- 

 tion. As evidence of the existence of a longitudinal division of 

 the chromatin thread and of the sequence of the two divisions, 

 I do not see how more could be asked of any material. In the 

 early prophase the greatly elongated and granular thread be- 

 comes twice split, once along its length and once across it. As 

 the cell ages, a continuously closer approximation of the chro- 

 momeres occurs, without obliterating the lines of separation 

 between the four parts of the segment ; accompanying this, the 

 segment becomes shorter and thicker, and the previously ex- 

 isting linear arrangement of the chromomeres is superseded. 

 AVhen the segments have reached approximately the size of the 

 definitive chromosomes of the metaphase, the nuclear mem- 

 brane disappears and distinction between cytosome and nu- 

 cleus is lost. As a coincident step, the formerly granular 

 segments become homogeneous in structure by the disappear- 

 ance of the chromomeres as individual structures ; all lines of 

 separation between parts are lost to view, so that an examina- 

 tion of the formed element would betray no indication of com- 

 posite structure. But, having traced the formation of the 

 chromosomes in this way, one is at no loss to identify each part 

 of the preexisting quadripartite chromatin segment. This is 

 possible because, while all trace of internal structure is gone, 

 the general outline is retained and the crosses and rings of the 

 early stages are still, even up to the metaphase, crosses and 

 rings. 



Having traced the formation of the ordinary chromosomes 

 through the various stages of the prophase, I should like to re- 

 turn to the beginning again and bring up to a like degree of 

 development the aberrant element which I have called the ac- 

 cessory chromosome. This has already been given in general 

 outline in my first paper upon Xiphidium (16), but a number 

 of important observations since made render a general discus- 

 sion desirable. 



I have not yet found it possible to make a detailed study of 

 the spermatogonia of the Locustids, as was done for the Ac- 

 rididse by Sutton in this laboratory, but sufficient observations 

 have been made to be assured that the accessory chromosome 



