470 
PHYSICS: A. W. HULL 
The possibility exists, however, that definitely planned experiments 
may enable us to regulate time and dosage of hormones better than is 
done in this experiment of nature; the results of such experiments can- 
not of course be foreseen. Nor can it be predicted in advance what the 
results of the inverse experiment might prove to be, i.e., treatment of 
the male zygote from the beginning of six-differentiation with female 
hormones. Such experiments will be necessary for the full solution of 
the stated problem. We can, however, state, confidently on the basis 
of the present results that sex-determination in mammals is not irre- 
versible predestination, and that with known methods and principles 
of physiology we can investigate the possible range of reversibility.^^ 
1 Steinach, E., Zentralhl. Physiol, B., 27, 1913, (717-723). 
2 Lillie, F. R., Science, New York, N. S., 43, 1916, (611-613). 
' Hunter, J., Account of the Free-Martin: Observations on Certain Parts of the Animal 
Economy. London, 1786, sold at no. 13 Castle Street; Leicester Square, pp. 45-68. 3 plates. 
(See also Atlas attached to Palmer's Ed. of Hunter's Works.) 
4 Spiegelberg, O., Zs. rat. Med., (Ser. 3), 2, 1861, (120-131, Taf. H). 
6 Hart, D. B., Edinburgh, Proc. R. Soc. 30, 1910, (230-241, 2 plates). 
^ Bateson, W., Problems of Genetics, (see pp. 44-45), Yale University Press, 1913. 
7 Cole, L. J., Science, New York, N. S., 43, 1916, (177). 
8 Chapin, C. L., /. Exp. Zool., Wistar Inst., Philadelphia, 1917 (in press). 
9 Goldschmidt, R., Amer. Nat., Lancaster, Pa., 50, 1716, (705-718). 
" Pearl, R., and Surface, F. M., Ann. Rep. Maine Agric. Exp. Sta., Oram, 1915, (65-80). 
" A full account of the work is in press in Journal of Experimental Zoology, 23, No. 2. 
THE CRYSTAL STRUCTURE OF MAGNESIUM 
By A. W. Hull 
RESEARCH LABORATORY. GENERAL ELECTRIC COMPANY. SCHENECTADY 
Communicated by R. A. Millikan. May 29. 1917 
Magnesium is assigned by crystallographers to the holohedral class 
of the hexagonal system, with axial ratio 1.624. The structure given 
below agrees with this symmetry. The arrangement of atoms is that 
of hexagonal close packing, the arrangement which equal hard spheres 
assume when closely packed, except that the structure is shortened 
by about one-half of one percent in the direction of the hexagonal axis. 
This is the fourth type of arrangement of atoms in elementary sub- 
stances thus far observed, viz.: the diamond type, characteristic of 
diamond, silicon, bismuth and antimony, where each atom is surrounded 
by four equidistant nearest neighbors; the centered cubic lattice, char- 
acteristic of iron, and probably also of nickel and the alkali metals, 
where each atom is surrounded by eight equidistant nearest neighbors; 
the face-centered cubic lattice, or closest-packed cubic arrangement, 
