1910.] NATURAL SCIENCES OF PHILADELPHIA. 319 



The longitudinal split of the postsynaptic stages which corresponds 

 to a similar split in the spermatogonia and remains open and traceable 

 until the chromosomes are upon the spindle might be assumed to be a 

 side-by-side union considered by A. and K. Schreiner (1904) to be 

 "the" method of conjugation, and as such I at first interpreted it; 

 but by a careful study of the steps in the formation of the chromosomes 

 from the segmentation of the spireme to the prophase, viz., the opening 

 of the chromatin segments into rings and their linin connection, it 

 becomes clear that the Weismannian method of reduction could only 

 be brought about if conjugation had taken place by an end-to-end 

 union, as first interpreted by Montgomery (1900), for only in this 

 way would there be one transverse and one longitudinal division, so 

 separating univalent chromosomes. A conjugation by parasynapsis 

 would result in two reduction divisions and the individuality of the 

 chromosomes would be destroyed. I am led to conclude that this 

 longitudinal split is a precocious division early laid down to mark the 

 plane of the second maturation division. 



The accessory chromosome first described by Henking (1891) for 

 Pyrrochoris apterus and later by McClung (1899), Sutton (1902), and 

 Baumgartner (1904) for Orthoptera, Blackmail for Myriopoda, Wal- 

 lace (1900) for Arachnida, Paulmier (1899), Montgomery (1898, 1901, 

 1906), Wilson, Stevens, etc., for Hemiptera, Stevens and Nowlin (1906) 

 for Coleoptera, and Lefevere and McGill (1908) for Odonata. has by 

 its peculiar behavior gone far toward establishing the theory of individ- 

 uality of the chromosomes. McClung was the first to suggest that 

 the accessory chromosome might be a sex determinant, believing 

 that this chromosome was peculiar to the sperm; but Stevens and 

 Wilson, while corroborating this suggestion of sex determination, 

 showed by a comparison of the equatorial plate of somatic cells and 

 germ cells of both sexes that it is the female and not the male that 

 possesses this additional chromosome. 



In his arrangement of the Heteroptera into three groups according 

 to the three types of spermatozoa, Wilson has brought all cases into 

 harmony with the dimorphism theory and has given direct evidence 

 of the conjugation of maternal and paternal chromosomes. The first 

 class is one in which there is a single heterotropic chromosome resulting 

 in two classes of spermatozoa of which one-half possesses, one-half 

 lacks this element ; in the second class the male has the same number 

 of chromosomes as the female but possesses one large and one small 

 idiochromosome while the female possesses two large chromosomes ; 

 and in the third class the idiochromosomes are equal in size in both 

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