THE ACCESSORY CHROMOSOME. 45 



thought advisable to give here all the probable references to the 

 accessory chromosome in the language of the different authors. 

 It may be possible thus to unify the conflicting opinions regarding 

 the essential features and to leave the field clear for a discussion 

 of the more involved points. Also, in furtherance of the plan 

 previously mentioned of confining the question to a limited area, 

 only the references to insects will be discussed in detail. By 

 this restriction, however, very little material will be excluded 

 since most of the work has been done upon insect testes. 



In the following paragraphs will be found all the references 

 in the literature at my command to structures which I regard 

 as possibly identical with the accessory chromosome. These 

 will be given chronologically and finally compared and com- 

 mented upon. 



1. The work of Plainer ('86) upon the Lepidoptera unfortu- 

 nately sheds no light upon the nature of the accessory chromo- 

 some in that order of insects. Only casual mention is made of 

 the nucleolus and the references do not enable us to gain much 

 insight into its character. The following quotations embrace the 

 principal references : " An letztern Punkten haben auch die Nu- 

 cleolen ihre Lage. Diese erscheinen selten in der Einzahl, meist 

 findet man deren zwei. Sie sind von ziemlicher Grosse, farben 

 sich starker und zeigen eine kugelige Form ; doch geben meist 

 die von alien Seiten an sie herantretenden Kernfaden ihnen ein 

 unregelmassiges zackiges Ansehen." 



2. " Diese (Elemente) farben sich gleichmassig stark mit Saf- 

 ranin und zeigen nicht nur in ihren Dimensionen sondern auch 

 hinsichtlich ihrer Zahl grosse Schwankungen. LJnter ihnen be- 

 finden sich auch die Nucleoli." 



3. Henking ('90) finds in the spermatogonia of Pyrr/wcoris, in 

 preparations fixed with picro-acetic acid, a nucleolus which is not 

 apparent in those preserved with Flemming's fluid. It appears 

 rounded and of some size, and retains a yellow color while the 

 chromatin stains dark red with carmine. It is observed occasion- 

 ally to have divided. 



4. Concerning the "nucleolus" of the first spermatocyte, 

 Henking has this to say : " Vor allem auffallig ist es an diesem 

 Zellen dass ein grosser Nucleolus zur Ausbildung gekommen 



