464 ZOOLOGY: F. R. LILLIE 
only when the compound capable of tautomerization is in the liquid 
phase, or in solution. And that is just the case with the free radicals 
under consideration. 
6. The two radicals, a-naphthyl-diphenylmethyl and a-naphthyl- 
xanthyl, were found to give a molecular weight less than that cal- 
culated for the monomolecular phase. Schlenk and Renning's^^ 
results show that a-naphthyl-biphenyl-phenylmethyl exhibits the 
same unusual behavior. We have good reasons to believe that our 
results are not due to experimental errors, and we took pains to verify 
them repeatedly. The cause of this unexpected result may possibly lie 
in a still further dissociation of the triarylmethyl itself, R R' R" 
RR'C + R''. 
1 Gomberg, Berlin, Ber. D. Chem. Ges., 40, 1907, (1860); 42, 1909, (406); Gomberg and 
Cone, Liebigs Ann. Chem. Leipzig. 370, 1909, (190); 376, 1910, (208). 
2 Gomberg, Berlin, Ber. D. Chem. Ges. (406); 46, 1913, (228). 
3 Compare G. N. Lewis, these Proceedings, 2, 1916,(588). 
* Gomberg and Cone, Berlin, Ber. D. Chem. Ges., 37, 1904, (2037). 
^ Schlenk and Mair, Liebigs Ann. Chem., Leipzig, 394, 1912, (179). 
6 Schmidlin, Das Triphenylemethyl, Stuttgart, 1914, (94). 
^ Piccard, Liebigs Ann. Chem., Leipzig, 381, 1911, (347). 
8 Schlenk, Ibid., 372, 1909,(4); 394, 1912, (186); Berlin, Ber. D. chem. Ges., 43, 1910, 
(1756); Schmidlin and Garcia-Banus, Ibid., 45, 1912, (3176). 
^ Schlenk and Renning, Liebigs, Ann. Chem, Leipzig, 394, 1912, (189). 
10 Schlenk, Herzenstein and Weickel, Berlin, Ber. D. chem. Ges., 43, 1910, (1754). 
11 Schlenk and Renning, Liebigs Ann. Chem., Leipzig, 394, 1912, (195). 
SEX-DETERMINATION AND SEX-DIFFERENTIATION 
IN MAMMALS 
By Frank R. Lillie 
DEPARTMENT OF ZOOLOGY. UNIVERSITY OF CHICAGO 
Read before the Academy, April 17. 1917 
The principle of zygotic sex-determination is generally regarded as 
established for mammals as for other animal groups. The reasons for 
this are (1) the identity of sex of all individuals derived from a single 
zygote: e.g., identical twins, quadruplets of armadillos, etc.; (2) the 
facts of sex-linked inheritance, which demonstrate the inheritance of 
certain sex factors in a Mendelian way; (3) the dimorphism of sper- 
matozoa in mammals as in other groups with zygotic determination of 
sex. We must therefore regard sex as determined, in the usual sense of 
the word, at the time of union of the gametes. 
The question, however, arises, whether sex-determination involves 
an irreversible tendency to the corresponding sex-differentiation, or 
