9 o THE MEIOTIC PHASE [CH. vi 



longitudinally, part of one longitudinal half of one chromo- 

 some may exchange places with a similar part of the cor- 

 responding longitudinal half of the other. In consequence 

 of this the heterotype division does not separate whole 

 chromosomes, for while the parts of the chromosomes that 

 have not exchanged places separate completely, the parts 

 of which the longitudinal halves have exchanged places in 

 reality only undergo a longitudinal division in the hetero- 

 type division, and therefore the second (homotype) division 



FIG. 13. Diagram illustrating JANSSENS' hypothesis of "Chiasmatypy." 

 (From JANSSENS.) 



a. Two homologous chromosomes, AB and ab, paired and crossing each 

 other, b. Each is longitudinally split, and one longitudinal half of 

 portion B of one has become attached to portion a of the other, while a 

 corresponding longitudinal half of b has become attached to A. c. Split 

 chromosome in the anaphase of the first (heterotype) maturation division; 

 one longitudinal half consists of portions ab, the other of aB. 



The complete segregation of AB from ab into separate nuclei will only be 

 effected in the succeeding homotype division which separates these longi- 

 tudinal halves. 



is needed in order to complete the separation. Reference 

 to Text-fig. 13 will make his scheme clearer. He points 

 out that this scheme not only explains the curious cross- 

 shaped figures, which may show twisting at one end and 

 not at the other, that are often seen in the heterotype 

 division, but also accounts for the universal presence of 

 two divisions in the maturation of the gametes, for both 

 divisions are required in order completely to separate the 

 homologous chromosomes into different nuclei. 



