192 PROCEEDINGS OF THE ACADEMY OF [Feb., 



and such as are unpaired or single in the spermatogonia and so do not 

 conjugate in the spermatocyte, which McChmg calls "accessory 

 chromosomes." Both these kinds agree essentially in their behavior 

 during the growth period of the spermatocyte, and are clearly distin- 

 guishable from the other ("ordinary") chromosomes by their compact 

 form and smooth outline ; they differ with regard to the point of being 

 single or double in the spermatogonia. Both kinds may occur in the 

 same animal, as I have shown (1901b) for Protenor. In Anasa I found 

 a pair of heterochromosomes in the ovogonia exactly like those in the 

 spermatogonia, which suggests that the paired heterochromosomes will 

 be found to occur in both maternal and paternal germ cells of the same 

 species; but whether unpaired heterochromosomes occur in maternal 

 germ cells is not known. 



Heterochromosomes that are paired in the spermatogonia and unite 

 to form bivalent ones in the spermatocytes I have described (1898, 

 1901a, 1901&, 1904a) for some forty species of Hemiptera, and in the 

 present paper for Lycosa (a spider) and Syrhula (an Orthopteron) ; 

 Henking (1890), Paulmier (1899) and Gross (1904) likewise for Hemip- 

 tera; and McQUll (1904) for Anax (an Odonate). Heterochromosomes 

 that are single in both spermatogonia and spermatocytes for Orphania 

 and Gryllus by de Sinety (1901), for Protenor by me (1901a), for Xiphi- 

 dium by McClung (1902), Brachijstola by Sutton (1900, 1902), and 

 Gryllus by Baumgartner (1904). 



Not to be confused with heterochromosomes are the "odd" chromo- 

 somes I described (1901a, h) for Alydus, Harmostes and (Edancola, 

 chromosomes that seem to behave exactly like any ordinary chromo- 

 some during the growth period of the spermatocytes, and cannot be 

 distinguished from them by any compactness of structure or intensity 

 of stain, except that they do not form bivalent chromosomes by conju- 

 gation with others. I called them odd because in cases where they are 

 present the spermatogonium has an odd or uneven number of ordinary 

 chromosomes, and the odd one is that which does not have a homologous 

 mate with which to pair during the synapsis stage. These resemble in 

 certain respects the unpaired heterochromosomes, but differ in not 

 maintaining a compact form during the growth period. These three 

 genera of Hemiptera are the only known cases where there is an uneven 

 number of chromosomes in the spermatogonia, without the odd chro- 

 mosome being a heterochromosome. 



And now we come to the point of the behavior of the heterochromo- 

 somes and the odd chromosomes during the maturation mitoses. 

 With regard to the heterochromosomes (chromatin nucleoli) that occur 



