INSECT SPERMATOGENESIS 603 



for example — in which the ultimate structure is clearly a con- 

 bination of plate-work and vesicles, the intermediate steps being 

 obscured by the less favourable structural features of the 

 chondriosomes. 



The next step in the condensation of the nebenkern is the 

 withdrawal of the chromophilic substance from all contact 

 with the periphery of the chromophobic mass as a whole, 

 resulting in the formation of a clear zone enclosing the now 

 centrally located chromophilic substance. This condition was 

 described by Platner (1889), and will be recognized as a con- 

 stant feature of the nebenkern in insects of all kinds. In accor- 

 dance with the view elsewhere developed (Bowen, 1922 ft), that 

 all this early activity in the nebenkern is merely indicative 

 of a centripetal condensation of the chromophilic material, 

 I would interpret the complete withdrawal of the chromophilic 

 material from the outer boundary of the nebenkern as merely 

 the last step in the progressive withdrawal of this material, 

 first from the outer periphery of the chromophobic mass as 

 a whole, and then from the connecting pathways which at 

 first traverse the outer chromophobic zone. 



The stages in this process of withdrawal have been omitted 

 by Gatenby so far as I can judge from his figures, and in my 

 own preparations I have been unable to get completely satis- 

 factory illustrative material. The process is undoubtedly 

 difficult of analysis because it is during this interval that 

 the rearrangements are completed which lead to the final 

 organization of the chromophilic material into a more regular 

 plate-work. A frequent appearance of this stage has been 

 figured by Meves (1900, fig. 67) in Pygaera, and is shown 

 still more clearly by Doncaster and Cannon (1920, figs. 20 and 

 21) in the louse. The central area of the nebenkern tends to 

 stain more or less completely (as a result of slight imperfec- 

 tions in technique), concealing the detailed arrangements of 

 the chromophiHc material, while from this central accumulation 

 delicate connexions pass out to the periphery of the nebenkern. 

 These connexions are gradually withdrawn and the disposition 

 of the chromophilic material now becomes progressively clearer. 



