THE DIPLOID CHROMOSOME COMPLEXES OF THE PIG 163 



phase (figs. 1, 2 and 3) and also in anaphase (fig. 16). With the 

 counts in these three stages checking, I feel confident of the 

 accuracy of'the result. 



The structure of the chromosomes 



Figures 1 to 20 show that there is some variation in the width 

 or diameter of individual chromosomes, in any given cell. The 

 difference within the cell is very slight and may be due to the fact 

 that the dye is extracted more rapidly from small than from 

 larger chromosomes. In more heavily stained material this dif- 

 ference is not so obvious (figs. 4 and 5) but exists to some extent. 

 The prophases do not show this variation in diameter. In some 

 of the metaphase chromosomes I am inclimed,to believe that the 

 extra width is due to the beginning separation of the chromatids, 

 the split being filled with dye. It will be noticed, however, that 

 the diameter of any one chromosome is the same throughout 

 its length. 



The presence at various stages of splits in the chromosomes 

 is so variable that at present definite conclusions cannot be 

 drawn. Iij . material passed through paraffin splits are occa- 

 sionally visible in a few chromosomes in prophases (fig. 3) and 

 it is not possible to determine whether these are obliterated be- 

 fore metaphase is reached or not. From studies of unsectioned 

 amnion which was much superior to the imbedded material for fine 

 details, it seems certain that all the chromosomes of the prophase 

 are split (fig. 81) and that this condition is maintained through 

 the metaphase. Slight differences in fixation or staining would 

 tend to obliterate these fine separations. A slightly oblique 

 view of the chromosome would also tend to obscure the split. 



In recent years the point of spindle fibre attachment has been 

 found important. It is difficult in the material I have of the 

 pig to directly determine this point as the spindles are visible in 

 but very few instances. Indirectly it is possible to obtain some 

 evidence as to the point of fibre attachment from the shape as- 

 sumed by the chromosomes in anaphase as the form at this time 

 would be dependent upon the point of the chromosome at which 



