1911.]  Gametogenesis of the Gall-Fly, N. lenticularis. 485 
of the nucleus is similar to the former. Instead, however, of sinking in, 
it divides at the surface by a division perpendicular to the edge, producing 
an irregular outer group of chromosomes (first polar nucleus) and an inner 
group: of parallel rod-like chromosomes. The latter divide immediately, 
apparently transversely, into an inner group which forms the egg-nucleus, 
and an outer or second polar group. The first polar group may divide 
into two. In the early segmentation mitoses of eggs of this class the 
haploid number (10) of chromosomes is found, and in complete series of 
sections of such eggs a double or triple group of polar chromosomes is always 
found at the edge of the egg. 
4, Since it is known that some parthenogenetic individuals lay eggs 
which all develop into females, and others lay only male-producing eggs, 
and since the female shows the diploid chromosome number in all its cells, 
while the male has the haploid number in the spermatogonia and nerve-cells, 
it is suggested that the eggs which undergo no maturation division become 
females, those which undergo reduction males. 
5. In the later segmentation stages of some eggs, and commonly in the 
yolk-nuclei of eggs which have reached the blastoderm stage, phenomena 
are described which are interpreted as indicating amitotic division. 
6. The haploid mitotic figures in the nervous system of the male, referred 
to previously, are more fully described, and a case is mentioned of a mitosis 
in a developing muscle-cell with about three times the normal (diploid) 
number of chromosomes. 
General Considerations. 
The facts set forth above confirm the conclusions with regard to sex- 
determination in Neuroterus which were drawn provisionally in the first 
part of this paper. There is no need, therefore, to discuss them further. 
I attempted, however, in my previous discussion to link the phenomena of 
sex-determination in Neuroterus on to. the results obtained by various 
methods in other groups of animals, and formulated a general scheme of 
sex-production, on the lines of Mendelian heredity, in which Neuroterus 
formed a special case. This scheme consisted in the assumption that in 
general the female contains female and male sex-determinants, represented 
by the symbols ¢ and 4, which segregate in oogenesis so that two kinds 
of eggs are produced, bearing @ and <¢ determinants respectively. The 
male contains only the ¢ determinant, but is heterozygous in. the sense 
that in its unreduced germ-cells only one ¢ determinant is present, which 
goes into half the spermatozoa, the other half of the spermatozoa containing 
no sex-determinant. This was expressed by representing the male as ZO, 
