DEPARTMENT OF GENETICS. 107 



tional workers will find it advantageous to work at the Station for 

 Experimental Evolution; but some of them may be able to carry on 

 parts of the work elsewhere. 



The Cytology of Datura Mutants. 



Especially valuable has been the assistance in this work given by 

 Mr. John Belling. He has perfected a new method for the examina- 

 tion of chromosomes in pollen-mother cells which avoids the tedious 

 method of cutting paraffine sections, and apparently renders possible 

 more accurate results than have hitherto been possible in counting 

 chromosomes. This method consists of the rapid fixing and staining 

 of the pollen-mother cells (freshly squeezed from the anthers) with 

 iron-aceto-carmine. These cells are caused to separate and then to 

 flatten out, by the aid of gentle pressure, after they have been for 24 

 hours in the reagent. This method has proved eminently suitable for 

 the purpose of counting and estimating the size of the chromosomes in 

 the second metaphases of chromosomal division. This method reveals 

 also the grouping of homologous chromosomes in the late prophases. 

 Mr. Belling finds, indeed, that in Datura the total number and assort- 

 ment of chromosomes in the mother plant are ascertained with greater 

 certainty bj^ counting both second metaphase plates in the same cell 

 than by counting the somatic chromosomes in the body tissue, say the 

 root-tip. By combining size estimations in the late first prophase and 

 in the second metaphase it is possible to recognize the particular chro- 

 mosome or chromosomes whose excess number characterizes the various 

 mutants. 



This specificity of the different chromosomes of Datura is a matter 

 of great importance for the analysis of the unbalanced chromosome 

 types. Though these 12 chromosomes look alike to the casual ob- 

 server, prolonged study shows that they fall into six grades of size: 

 1 largest, 4 large, 3 large medium, 2 small medium, 1 small, and 1 small- 

 est. In triploid Daturas the 36 chromosomes are in 12 groups of 3 

 each. In tetraploid Daturas the 48 chromosomes are in 12 groups of 

 4 each, each group forming a figure 8. In triploid and tetraploid all 

 of the 3 or 4 chromosomes in each group are of the same size. In 

 simple trisomic plants there are 11 ordinary paired chromosomes and 

 1 trivalent, like the trivalents of the triploid plant. In the mutant 

 Globe all three chromosomes are small. In the mutant Mutilated 

 all three are large. 



In triploid plants, at the early anaphase of the first division, 2 

 chromosomes of each trivalent go toward 1 plate and 1 chromosome 

 toward the other. In the normal diploid Datura, non-disjunction 

 occasionally gives rise to an assortment of 11: 13 in the second meta- 

 phase stage. This non-disjunction was seen once in 100 cells observed. 

 In the higher "balanced types" of excess chromosomes, the metaphase 



