INSECT SPEEMATOGENESIS 599 



especially to corroborate Gatenby's statement ^ of the duplex 

 structure of these mitochondrial spheres, since upon this 

 point depends a proper understanding of the formation of the 

 nebenkern. Each sphere consists primarily of a droplet of 

 some substance which has little or no affinity for the usual 

 stains, and to which the name of chromophobic material has 

 been appropriately applied. This material is enclosed in 

 a delicate envelope of some substance which takes haemato- 

 xylin rather sharply, and is accordingly termed the chromo- 

 philic substance. I wish to call special attention to the fact 

 that in a surface view of one of these mitochondrial vesicles 

 this chromophilic layer is so delicate that as a rule it does not 

 appreciably affect the transparency of the vesicle as a whole. 

 Only around the periphery of the sphere, where the thickness 

 of chromophilic material is sensibly increased by the effects of 

 curvature, does it become clearly visible. In other words, 

 a single (or double) thickness of chromophilic material would 

 not be noticeable in a properly differentiated preparation. 

 This point should be clearly understood, since upon the optical 

 principle involved depends a proper interpretation of the 

 ' spireme ' in the nebenkern. 



In the maturation divisions the mitochondrial vesicles seem 

 rarely to retain their spherical shape, but, as Gatenby has also 

 noted, are usually more or less drawn out in a direction parallel 

 to the long axis of the spindle. This is sometimes so pronounced 

 that in a cross-section through the region of the spindle poles 

 the vesicles, closely packed and decidedly elongate, are seen 

 to radiate outward from the neighbourhood of the centrioles, 

 reminding one very strongly of the conditions which I have 

 described in the Hemiptera (Bowen, 1920). Something of 

 this same appearance is shown by Gatenby (1917 a) in his 

 fig. 48, which is especially interesting because the nucleus is 

 still in the prophase, with its membrane intact. As the groups 

 of daughter chromosomes separate during the anaphase, the 

 mitochondrial vesicles become drawn out along the spindle, 



^ This structure of the mitochondrial vesicles was first correctly described 

 by Meves (1900). 



