488 Mr. L. Doncaster. ; [Jan. 25, 
female characters to appear. This hypothesis of potential hermaphroditism of 
both sexes, combined with special ¢ and 2 activators, would thus allow of 
the occasional modification of sex by environment, if the stimuli supplied 
were able to bring about the same physiological effects as the inherited 
Gg and ? determinants. That environment may have some such effect in 
modifying somatic characters which are inherited alternatively is indicated 
by the work of Tower,* and there seems to be no a@ prior reason for 
denying such an effect in the case of sex. 
EXPLANATION OF FIGURES ON PLATE 17. 
All the figure are freehand drawings, made with a Zeiss aprochromat. 3 mm., 1°40 n.a., 
and compensating ocular 12.) 
ies. 1—3.—Three stages of the approach of the nucleus to the edge of the egg, preceding 
maturation. 
Fries. 4, 5.—Prophases of first segmentation division ; diploid type of egg. 
Fie. 6 (a, b).—First segmentation division, same series as figs. 4, 5; a and b show the 
mitotic figure cut in two successive sections. 
Fic. 7.—Completion of first segmentation division ; same series as figs. 4—6. 
Fic. 8.—Anaphase group of segmentation mitosis, diploid type. Nineteen chromosomes 
are visible in this section; at the other end of the same spindle, two sections 
removed, 15 or 16 can be counted. | 
Fie. 9.—Prophase of diploid segmentation division, showing about 20 chromosomes 
arranged in irregular meridional bands under the nuclear membrane. 
Fie. 10.—Late prophase, same series as fig. 8, showing intranuclear origin of spindle. 
Fre. 11.—Prophase of polar division, haploid type of egg. 
Fie. 12.—First polar division, nearly completed. The division figure ran obliquely 
through two sections; all the inner part is drawn from one section, the greater 
part of the outer group of chromosomes from the next. 
fic. 13 (a, b).—Second polar division; the division figure runs obliquely through two 
sections. a represents the separation of the chromosomes of the egg-nucleus from 
the second polar chromosomes ; 6, in the next section, the outer polar group. 
Fic. 14 (a, b).—Two successive sections, showing completion of second polar division. 
In a, egg-nucleus on right, second polar group in middle, part of division spindle 
of outer polar group on left ; 6, division-figure of outer (first polar) group. 
fie. 15.—Anaphase group of segmentation division, haploid type. 
Fic. 16 (a, b).-—-Anaphbases of haploid segmentation divisions; a, cut across; 6, seen 
sideways. Both from the same egg. 
Fic. 17 (a, b).—Two late anaphases, haploid type, from the same egg. In a only the 
chromosomes at the upper side of the figure are shown. In 6 all the chromo- 
somes (5) visible in the section are shown ; the other end of the figure was broken. 
Fic. 18.—Group of polar chromosomes during segmentation stages ; same egg as fig. 15. 
To the left, a large group of about 20 (outer polar group); to the right, smaller 
inner polar group. 
* © Biological Bulletin,’ 1910, vol. 18, p. 285. 
