102 



PHYSICAL BASIS OF HEREDITY 



There are certain questions connected with crossing 

 over that are illustrated by the following models (Figs. 

 41, 42, 43) . In these models of tetrads the dotted rod, split 

 lengthwise, stands for a maternal chromosome, and each 

 of its halves^ may be called a strand. The split in the rod 

 is the secondary (or equational) split. The black rod, 

 also split lengthwise, stands for the paternal chromosome. 



In Fig. 41, a, the two split rods are represented as 

 twisted about each other. If the two inner strands break 

 and the cords interchange at the levels, where they first 

 come into contact with each other (Fig. 41, &), and then 



FlQ. 42. — Scheme showing the opening out of the strands of the tetrad, a, in two planes. 

 b, according to Robertson and Wenrich. 



later the four strands come to lie side by side, i.e., "fuse," 

 the result will be that shown in Fig. 41, c. Two of 

 the strands represent crossovers in the sense that an 

 interchange has taken place between a maternal and a 

 paternal strand ; and if at the first spermatocyte division, 

 when the threads begin to pull apart, the maternal sepa- 

 rate from the paternal threads, two threads may be seen 

 actually crossing each other (Fig. 41, d). They are here 

 the two non-crossover strands, but if the two strands 

 thrown to the left had been thrown to the right the two 

 crossover strands would cross over. The scheme is essen- 

 tially the same as the chiasma of Janssens, but the strands 

 that cross may or may not (as here) represent the cross- 

 over strands. 



