SPERMATOGENESIS OF STENOBOTHRUS VIRIDULUS. 3 



segregation of character factors necessary to that mode of inheritance : the 

 members of each spermatogonia! pair are assumed to be respectively paternal 

 and maternal in derivation, so that the juxtaposition of their component 

 chromomeres permits the exchange of character factors obtained from the 

 two parents. This is merely an hypothesis, but there seems to be little 

 doubt that the number and size and shape relationships of the chromosomes 

 are constant for the species ; and it is probable that we shall eventually find 

 morphological correlation between the complexes of allied members of a 

 group. 



Lastly, there is the problem of the heterochromosomes, investigated 

 originally by Wilson, and divided by him into three classes — idiochromo- 

 somes, heterotropic chromosomes, and microchromosomes. The first-named 

 consist of two elements, differing in size and staining deeply during the 

 resting stages and growth period of the primary spermatocytes ; they later 

 conjugate, and still later divide, the larger passing to one pole and the 

 smaller to the other. The oogonia show a corresponding pair of chromo- 

 somes, but in this case both are of the same si^e. Spermatozoa possessing 

 the larger idiochromosome produce females, those possessing the smaller 

 produce males. The heterotropic chromosome occurs in the spermatogonial 

 cell as a single element, and behaves like the ordinary chromosomes in the 

 second maturation mitosis, but passes entire to one daughter cell at the first. 

 As in the case of the idiochromosome, it is represented in the oogonia by 

 a pair. Spermatozoa containing the heterotropic chromosome produce 

 females, and those without it males. Wilson has suggested that, in the male, 

 it acts as a male determinant, and that it passes from one sex to the other 

 alternatively, being recessive in the female : Hertwig, Paulmier, and Wassilieff 

 regard it as a degenerating chromosome that will eventually become extinct — 

 a view strongly opposed by McClung. 



In 1899 McClung drew attention for the first time to this peculiar 

 chromosome in the male germ-cells of XipUdium ; and it has since been 

 studied in a large number of organisms, particularly Orthoptera. He 

 found that it undergoes no resolution into a spireme during the primary 

 spermatocyte resting-stage, but persists as a compact and darkly staining 

 body on the periphery of the nucleus : he erroneously stated that it divides 

 longitudinally at both maturation divisions, but corrected this mistake in a 

 later paper upon the Locustidai. This " accessory '' chromosome of McClung 

 has been found by de Sinety in the Phasmidse, and by Sutton in Bracliystola : 

 Baumgartner has studied it in Gryllxis ; and his results have been confirmed 

 by Gutherz, working upon the same material. Otte has observed it in 

 Locusta, Gevsivd in Stenohothrus biguttulus, '^owlin in Mela7ioplus bivittatus, 

 and Robertson in Syrbula admirabilis : Davis has seen it in every member 

 of the Acrididse and Locustidse that he has studied, and further, has 

 phown that this " monosome " is represented in the oogonia by a pair of 



X* 



