EXPLANATION OF PLATES XXI, XXII. 



The twelve chromosomes of each cell are arranged in a series or row 

 horizontally. The series are placed one below the other, so that like 

 chromosomes always appear in the same column. The number (at top 

 of page) of the column in which a chromosome occurs is the number by 

 which the particular individual chromosome which that column contains 

 is knov^m. The series or cell complex to which a chromosome may belong 

 is indicated by the numbers at the left side of the plate. The capital 

 letters at the left side of the plate designate the animal from which the 

 series was taken. The characters XX indicate the proximal part of the 

 chromosome, the point at which the spermatogonial pair conjugated. 

 They also indicate the points of attachment of the spindle fibers, and in 

 those chromosomes that are part way advanced into mitosis the ends of 

 the transverse axis. In the second spermatocytes and the spermatogonia 

 they indicate, as above, the attachment of the fibers and the proximal 

 or polar end of the chromosome. 



The first twenty-five series are taken from first spermatocyte meta- 

 phases, such as are seen in figures 30-33. Series 26-29 were taken from 

 second spermatocyte metaphases, such as figures 40 and 41 (Series 28 is 

 from fig. 41, plate XX) ; and series 30 and 31 were taken from the sper- 

 matogonial metaphases shown in figures 2 and 6 of plate XVIII. All of 

 the series except 24 and 25 are from cells that were stained with iron- 

 haematoxylin. Series 24 and 25 were stained with Flemming's tricolor, 

 which has a tendency to swell the structures. The magnifications are the 

 same as in the preceding plates and the drawings were made in the same 

 way. 



