158 M. \V. IU.ACKMAN. 



EXPLANATION OF PLATE I. 



All drawings were made by the author with the aid of a camera lucida. The 

 optical equipment consisted of a Zeiss apochromatic objective of 2 mm. focus and 

 a number 12 compensating ocular, the source of the light being a Welsbach mantle 

 The original magnification was 2,300 diameters and the drawings were reduced 

 one fifth in reproduction, making the final magnification 1,840 diameters. 



The seventeen chromosomes arranged in each horizontal row represent the 

 chromosomes of a single cell, as seen in a side view of the spindle in the metaphase. 

 They are arranged as follows: the six which show the characteristic cruciform shape 

 comprise the first six of each row and are lettered A, B, C, D. E and F. Those 

 showing the double-V shape five in number are lettered G, H, I. J and K. 

 Those corresponding to the double-rod type of structure are lettered L, M, N, O 

 and P. The seventeenth and last chromosome in each row is the accessory and 

 is distinguished by the letter Q. The individuals of each type of structure are 

 further arranged in a graded series as regards size, the largest first, etc. 



Figs. 1-15 represent the chromosome groups in the metaphase of the large type 

 of first spermatocytes. 



