7 6 



Edmund B. Wilson 



It is evidently this pair that give rise to the bivalent "accessory" 

 (eccentric) chromosome of the first division and hence to the 

 chromosome nucleolus of the growth period. Gross correctly 

 describes this bivalent as a quadripartite body or tetrad, but 

 overlooked the fact that it is composed of two slightly unequal 

 halves, and these correspond in relative size to the unequal pair 

 in the spermatogonia. This appears unmistakably in a great 

 number of polar views of the first division metaphase (though it 

 is not always apparent) and is clearly shown in Photos 3, 4 and 

 <J. It is evident that the bivalent is so placed in the equatorial 



a 



I 



FIG. I. Four spermatogonial chromosome-groups of Syromastes marginatus; a and b are reproduc- 

 tions of Photos i and 2.* 



* The drawings are not made from the microscope with the camera lucida but directly upon enlarged 

 photographs of the objects. Since I believe this method to be superior in accuracy for the representa- 

 tion of such small objects I will briefly describe it in the hope that others may find it useful. The 

 original negatives are taken directly from the sections at an enlargement of 1500 diameters (2 mm. oil 

 immersion, compensation ocular 6). From these negatives enlarged bromide prints are made (with a 

 photographic camera) three times the size of the original negatives (i. e., 4500 diameters) upon double 

 weight paper, which gives a good surf ace for pen drawings. The drawing is then made directly on the 

 print with waterproof ink, and when thoroughly dry the remains of the photograph are bleached out in 

 a mixture of sodium hyposulphite and potassium ferricyanide. The enlarged prints of course show the 

 chromosomes with more or less blurred outlines (though they are clearer than might be supposed); 

 but by working with an ordinary print and the object before one for comparison the drawings may 

 nevertheless be made with great accuracy. They may be tested and if necessary corrected, by the use 

 of a reducing glass. 



