THE ACCESSORY CHROMOSOME. 57 



her of chromosomes. All but one of these rapidly disintegrate 

 and from their substance produce the spireme of the first sper- 

 matocyte. One persists in its original form and, assuming a pe-' 

 ripheral position, continues to stain as does a chromosome of the 

 metaphase. During metakinesis it is divided like other chromo- 

 somes. This is the accessory chromosome. 



48. " The accessory chromosome is a constant and important 

 element of the germ cell. It arises, in the Acrididae, from a 

 spermatogonial chromosome, and from that time forward main- 

 tains a separate and distinct existence. During the prophases, 

 when occur the profound changes that result in the production 

 of a nucleus with only half the ordinary number of chromo- 

 somes, this structure stands aloof and self-contained. With the 

 establishment of the mitotic figure of the first spermatocyte, how- 

 ever, it takes its place with the other chromatic elements and be- 

 comes indistinguishable from them henceforth until the spermatids 

 are formed. Here it again becomes distinct and conspicuous." 



49. Up to the present time, we have learned nothing of the 

 real origin of the accessory chromosome. That it is one of the 

 spermatogonial chromosomes that passes over into the sperma- 

 tocytes without taking part in the formation of the spireme is 

 pretty well established ; but as to the manner of its appearance 

 in the spermatogonia, we have no knowledge. Sutton ('oo) is 

 the only one who has traced the history of the element at all 

 fully in the spermatogonia. Concerning its first appearance and 

 its subsequent history in this generation of sex cells he says, " In 

 Brachystola, the accessory chromosome appears probably in the 

 first, and certainly in the third, secondary spermatogonial division, 

 and goes through precisely the same changes in each cycle up 

 to the last. 



50. " It may occasionally be distinguished from the other 

 chromosomes in the metaphase and anaphases by its granularity 

 and greater length, though it always divides like the others, and 

 in the actual process of division, as a rule, is indistinguishable 

 from them. In the telophase it constructs its own membrane 

 just as the others do, but soon becomes sharply contrasted with 

 them by the deposition of its chromatin in a diffused condition 

 upon the inner surface of its vesicle (vesicular chromosome), and 



