298 



THE AMERICAN NATURALIST 



[Vol. L 



become much shorter and thicker than in the amphitene 

 stage, and each chromosome in the pair can in many prep- 

 arations be seen to have split lengthwise, i. e., the "tet- 

 rads" have formed preparatory to the two maturation 

 divisions. Janssens often finds the four threads placed 

 somewhat as shown in Fig. 8a, two of the threads crossing 

 at one or two points, but otherwise being rather widely 

 separated, and the other two threads rarely crossing but 

 lying close to whichever one of the two threads first men- 

 tioned happens to be on the same side, and merely bend- 

 ing inwards and then back again where the first two 

 threads cross. The peculiar crossing of two of the 

 threads and the bend in the other two, as shown at point 

 L, he interprets, in the way shown in Fig. Sb, as meaning 



that both pairs of threads originally were twisted across 

 each other, but that the two homologous threads which 

 were originally on the inner side, and so touched each 

 other, underwent recombination, i. e., "crossed over," at 

 the point of contact ; each of the new chromosomes thus 

 formed, therefore, would lie entirely on one side or the 



