In these latter stages the nucleolus has entirely faded out and nothing suggesting an accessory chromosome is present.
Thionin was found to be a very useful stain for distinguishing between the accessory chromosome and an ordinary nucleolus.
In Blattella we have a typical "accessory chromosome," according to McClung.
Of the insects considered in this paper Aphis and Termopsis have no "accessory chromosome" or "heterochromosome" of any kind.
A brief account of a chromatin element resembling the accessory chromosome in Sagitta has been added for comparison.
The accessory chromosome has been now observed in most of the great divisions of insects, except, as it happens, Lepidoptera.
Montgomery afterward showed that Henkings nucleolus was an accessory chromosome.