CONFIGURATIONS OF BIVALENTS OF HYACINTHUS. 487 



V's diminish in size as their chromatids are pulled out by the 

 spindle fibers towards the poles. 



(3) The numbers of cases of possible segmental interchange in 

 the chromatids of the large bivalents, calculated from the 

 numbers of nodes, agrees roughly with the numbers of cases of 

 crossing-over found in the first chromosome of Drosophila, which 

 is especially favorable for this study. 



Some process of segmental interchange seems demanded by 

 the genetic evidence in Drosophila, Zea, Lathy rus, and the other 

 plants and animals which have shown cases of crossing-over. 

 Hence it is apparently the natural scientific procedure to accept 

 segmental interchange as a working hypothesis to account for 

 the nodes and internodes of the chromosome pairs in the 

 Orthoptera and other animals, and also in Uvularia and Hya- 

 cinthus. This is the more imperative in that there seems no 

 other available working hypothesis. 



SUMMARY. 



(1) The four large bivalents of Hyacinthus show in 62 cases one 

 node, and in 54 cases two nodes where the homologues cross. 



(2) At these nodes it can apparently be seen with the micro- 

 scope that one chromatid of each homologue passes obliquely 

 across, while the other seems bent back along the other homologue. 



(3) The hypothesis of previous segmental interchange at such 

 a point is assumed until a better hypothesis is found. 



(4) The numbers of chromatids showing such points of seg- 

 mental interchange, according to the hypothesis, calculated from 

 the 116 bivalents examined, were 38 per cent, with no inter- 

 change, 50 per cent, with one point of interchange, and 12 per 

 cent, with two points of interchange. 



LITERATURE CITED. 

 Belling, J. 



'25 Homologous and Similar Chromosomes in Diploid and Triploid Hyacinths. 



Genetics, 10: 59-71. 

 '26 Single and Double Rings at the Reduction Division in Ui'idaria. BIOL. 



BULL., 50: 355-363. 

 Morgan, T. H. 



'25 The Theory of the Gene. Yale Univ. Press. 

 Janssens, F. A. 



'24 La chiasmatypie dans les insectes. La Cellule, 34. 



