THOS. II. MoMi.oMKRV, JR. 



A. Tin 1 h I'M] HKSIS. 



The fir-t -tatemeiit of the hypothesis is due to McClung 

 in [902, after in\ estimation of the unpaired accessory chronio- 



me in the spcrmatogein-si- of < )rthoptera llial kind of 

 modified chromosome named by me in 1906 the monosome. 

 Mi ('lung's conception of the function exerci-cd by llie accessory 

 chromosi mic is "that it is the bearer of those qualities which 

 pertain to the male organism, primary among which i- the faculty 

 of producing -ex cells that have the form of spermatozoa." He 

 i/cd al-o ih.it there must he selective fertilization, that 

 to the ovum "come the two forms of spermatozoa from which 

 -election i- made in response to environmental necessities." At 

 that time nothing was known of the maternal chromosomal num- 

 ber, so that it was natural for McClung to reason that the mono- 

 some \\as .1 pairnial chromosome not represented in the female. 



In the same year Sutton (1902) described for Bracliystola that 

 "twenty-three is the number of chromosomes in the male cells, 

 \\hile i\\rnty-two is the number I have found in the female cells, 

 and thn- \\ e -ecm to find a confirmation of McClung's suggestion 

 that the accessory chromosome is in some way concerned in the 

 determination of sex." Subsequent studies have shown that 

 Sutton \\as wrong in his count of the oogonial chromosomes. 



Then Stevens ! 11105) found in Tcnchn'n "that in both somatic 

 and germ cells of the two sexes there is a difference not in the 

 number of chromatin elements, hut in the size of one, which is 

 \vr\ -mall in the male and of the same size as the other nineteen 

 in the female. . . . The small chromosome itself may not he a 

 deierminant, but the conditions in Tcuchrio indicate that sex 

 may in some < as( - he determined by a difference in the amount 

 or (|ualil\ of i he chromatin in different spermato/oa." In K)O6 

 she wrote: "The scheme also assumes either select i\ e fertilization, 

 or, what amounts to tin- same ih ing, infertility of gametic unions 

 where like sex chromo-ome- are present"; and in H)o<)</ : "Tin- 

 only other alternative in these insects seems to be that sex \^ 

 already determined in ih. egg helon- fertili/ation, either as a 

 matter of dominance, or as a re-ult of maturation, and that 

 zation is selective . . . but any -uch general application 

 i- premature until adequate evidence i- at hand to prove thai 



