PLATE 4 



EXPLANATION OF FIGURES 



Studies on the organization of the first spermatocyte chromosomes. The 

 figures are all at a magnification of 5600 diameters. 



27 to 31 show the various forms of chromosomes present in the albino rat. 



27 Eighteen chromosomes drawn from one cell. A,B,C,D, particular chromo- 

 somes as in figures 18 and 19; Cr., cross; Nu., nucleolus; R, ring; A', accessory. 



28 A chromosome not seen in the cell from which figure 27 was made, but 

 found in a neighboring cell. 



29 A cross in midprophase (in a somewhat earlier stage than that of the 

 chromosomes of figure 27). 



30 to 33 Prominent stages in the history of chromosome A when it forms as a 

 triple ring; the subsequent stages are essentially similar to those shown in figures 

 38 to 42. The organization of this form is interpreted diagrammatically in figure 

 43. 



34 to 42 show the history of a large chromosome when it forms as a double 

 instead of a triple ring. 



36 A side view of figure 35. 



37 A similar view at a slightly later period; the chromatin is more condensed. 

 41 and 42 show the formation of the simple V's in anaphase. 



43 to 45 Drawings from McClung ('14) (original from Granata), showing my 

 interpretation of the organization of the simple and complex ring chromosomes. 



182 



