RESULTS OF CROSSING. 337 



EXPLANATION OF PLATES I.-III. 



We photographed each intromittent organ at a magnification of 20 diameters 

 and prints on matte paper were made from these negatives. Then each subject 

 was accurately measured with a small pair of architects' dividers fitted with no. 9 

 needle points, and set at 2 mm. The measurements were made on the matte 

 prints so that each division of 20 mm. could be indicated by a pencil mark and 

 numbered. The measurements were taken from the distal end of the intromittent 

 organ to the point where the thick part of the coil enters the gland. The coil is 

 most easily dissected off at this point (e. g., photos 3, 5, 6, 8, etc.) but in those 

 cases where part of the canal within the gland has been preserved (e. g., photos 

 i and 2) the point from which the measurement was made may be readily deter- 

 mined, for the part within the gland is transparent and quickly tapers to a finer 

 canal which apparently extends through the entire gland. The transparency of 

 that portion of the canal within the gland is much clearer in the preparations than 

 in the photographs, for when the chitin has a yellow tinge it appears black and 

 opaque in the photograph. 



In many specimens we have left a small piece of the gland at the point where 

 the intromittent organ enters (e. g., photos 7, 9, 10, 12, 14, 19, etc.). 



