668 C. E. McCLUNG 



homologous chromosomes are joined together, but occasionally 

 at the opposite end or between the extremities. All the features 

 enumerated are constants except the one involving the spatial 

 relations, and to the variations here occurring are due the diverse 

 forms of chromosomes found in the first spermatocyte. The 

 problem is therefore to determine how it is possible to derive from 

 a chromosome consisting of four similar chromatids, with con- 

 stant and uniform fiber insertion, the various rods, rings, crosses, 

 V's, double V's, K's, X's, U's, I's, 8's, and other less common 

 forms, without alteration of structure and merely by change of 

 the spatial relations of the four elements. 



A written description of these permutations is not easy, but 

 it is a very simple matter with a clay model to produce every 

 form to be found merely by changing the relative positions, of 

 its four parts. These changes fall into three general types of 

 movement: (1) flexures in the plane of the longitudinal cleft, 

 (2) movements of the chromatids in a plane normal to that of 

 the longitudinal cleft, (3) divergence of the four chromatids at 

 the center while the ends remain in contact. In addition to 

 these simple movements there are (a) combinations of these and 

 (b) rearrangements of the four chromatids resulting in various 

 complicated forms. As mitosis proceeds these combinations, 

 especially of the first (a) kind, become more common and in the 

 metaphase and anaphase, the rule. The point at which move- 

 ment occurs in classes (1) and (2) is at the center of the rod where 

 homologous chromosomes are joined together, and this fact can- 

 not be devoid of significance. 



The simplest relations are those found in the rod-shaped 

 chromosome where the four parts lie extended along one axis. 

 These I-shaped tetrads are commonly the smaller ones of the 

 complex and do not often occur without some enlargement at 

 their middle where the homologous chromosomes are fused. They 

 represent the final step in the divergence of the joined parental 

 chromosomes which, if there be an earlier parasynapsis, finds its 

 opposite in the side to side position of these units (fig. 117). 



Related to this simple form are those of the first category pro- 

 duced by flexure in the plane of the longitudinal cleft, and which 



