482 JOHN BELLING. 



should split into separate chromatids, while the vertical rings or 

 V's should divide into upper and lower halves, without separating 

 into chromatids; (5) the numbers of bivalents with one or two 

 nodes should permit of a calculation of the numbers of chromatids 

 with segmental interchange at no, one, or two points, which 

 should possibly more or less resemble the occurrence of no, 

 single, or double crossing-over in Drosophila. In such a calcu- 

 lation, the numbers of chromatids with no segmental interchange 

 would be equal to twice the cases of single nodes plus the number 

 of cases of double nodes. The total number of chromatids with 

 one point of interchange would be got by adding twice the number 

 of bivalents with single nodes to twice the number with two 

 nodes. While the chromatids with two points of interchange are 

 equal to the number of bivalents with two nodes. It should be 

 possible to test these five points. 



(It seems obvious that a junction of homologous chromosomes 

 at the ends has no relation with segmental interchange. It is 

 probably otherwise with junctions at the point of constriction of 

 the chromosome, where segmental interchange may well take 

 place.) 



It was for the purpose of testing this hypothesis that the present 

 study was made. 



LARGE BIVALENTS OF Hyacinthus. 



The variety of Hyacinthus orientalis investigated was one of 

 those formerly studied (Belling, 1925), and was chosen because it 

 could be readily identified by the flowers, and had marked 

 characters even in the bulbs. This was the diploid clone called 

 'Yellow Hammer." The bulbs were obtained in October, and 

 put into water during that month and the next. Division of the 

 pollen-mother-cells usually accompanied the development of the 

 first roots. The pollen-mother-cells were instantaneously fixed 

 by being squeezed out from the anthers into iron-acetocarmine. 

 The chromosomes were observed with Zeiss' water-immersion 

 objective 70, yellow-green light, and a water-immersion con- 

 denser. 



At the first metaphase in the pollen-mother-cells (Fig. i) the 

 four long chromosome pairs commonly assume one of six different 



