blackman: the spermatogenesis of scolopendka. 91 



B. The Accessory Chromosome. 



The literature upon the accessory chromosome has been reviewed 

 with such detail in a recent publication by McClung (:02 a ) that it will 

 not be necessary or desirable for me to consider it at any length. This 

 element has now been found in three classes of the branch Arthopoda, 

 viz., Insecta, Arachida, and Myriapoda. A structure in many respects 

 similar to it, and apparently representing it, has also been described 

 by Montgomery (:00) in the Protracheata (Peripatus). This chromo- 

 some displays a constancy of behavior in the spermatocyte divi* 

 sions of insects which argues strongly for its great importance. 

 This constancy is especially marked in the various families of Orthoptera. 

 The element in the cells of this group is derived directly from a 

 single spermatogonial chromosome — Sutton (:00, :02) in Acrididae ; 

 McClung (:00) in Acrididae and (:02 h ) Locustidae ; de Sinety (:01), in 

 various Orthoptera — and according to the last two authors mentioned 

 participates in but one of the maturation divisions. In these cells the 

 accessory chromosome seems always to maintain its own individuality, 

 and for this purpose it is at various stages in its early history contained in 

 a vesicle separate from the other chromosomic vesicles which go to form 

 the nucleus (Sutton, :00), while in the prophase of the spermatocyte it 

 is at all times plainly recognizable on account of its less granular nature. 

 For a considerable time it was supposed that it differed from the other 

 chromosomes of the same period by the entire absence of the spireme 

 condition. This conclusion is now found to be untenable, since during 

 the spermatocyte prophase in Orchesticus and other locustids, the acces- 

 sory chromosome assumes a spireme stage (McClung, :02 h ). The spireme 

 of this element, however, differs from that of the other chromosomes in- 

 asmuch as the chromatin is less diffuse, and in its early stages so closely 

 coiled upon itself that its true nature can be learned only by using 

 high magnification. Later this thread thickens and unfolds, at this time 

 resembling the ordinary segments in mid prophase, except in the fact 

 that it is more homogeneous. Thus it is seen that this element does 

 not differ so markedly from the other chromosomes as was at first 

 supposed. 



Regarding: the function of the modified chromosome, two theories have 

 now been advanced. Paulmier in his paper on Anasa puts forth the 

 theory that the " small chromosome " represents characteristics which 

 are being eliminated from the race. He bases these conclusions entirely 

 upon the failure of the element to divide in one of the spermatocyte 



