586 L. DONCASTER. 
really parthenogenetic in this case also, it must be supposed 
that the entrance of the spermatozoon causes a reversal in 
the arrangement of the polar nuclei, and so females result. 
And if the two inner nuclei of the four produced at the 
second polar mitosis differ from one another only in that they 
bear different sexes, and it is to some extent a matter of 
chance which one becomes the egg-nucleus, it suggests an 
explanation of the fact that in all the species observed the 
second polar nucleus breaks up into chromosomes, each of 
which may split, but never form a true mitosis. The nucleus 
has in it the power of further development, but some part of 
the division mechanism seems to fail, and after what looks 
like a fruitless effort to divide, it gradually disintegrates. 
In this respect it differs from the outer polar nuclei, which 
usually disappear at an earlier stage, and show much less 
tendency to divide. 
Zoo. LaB., CAMBRIDGE ; 
February, 1905. 
Since writing the above, I find that in Peecilosoma pul- 
veratum, which yields only females from virgin eggs, the 
maturation is exactly as in normal eggs of P. luteolum. 
In Hylotoma rosae (males from virgin eggs) the inner 
polar nuclei approach one another as in other male-producing 
species, but apparently disappear without breaking up into 
chromosomes. 
December, 1905. 
List OF PAPERS REFERRED ‘TO. 
1. CastLe, W. E.—‘‘ Heredity of Sex,” ‘Contrib. Zool. Lab. Harvard,’ 
No. 137, January, 1903, p. 189. 
2. CamEeron, P.—‘ Monograph Brit. Phytophag. Hymenoptera,’ vols. i and 
ii, Ray Soc., 1882-4. 
3. Gross, J.— Spermatogenese von Syromastes marginatus,” ‘Zool. 
Jahrbucher (Spengel) Anat. und Ontog.,’ xx, 1904, p. 439. 
