Sex- Determination in the Gall-Fly, N. lenticularis. 197 



nearly allied to Neuroterus), which may throw important light on the nature 

 of the peculiar mitotic figures found in the maturation divisions of Neuroterus 

 eggs. In the early oocyte, after a compact synizesis (contraction phase) a 

 number of thread-like chi'omosomes emerge from the spireme, appear to pair 

 end to end, and the pairs then arrange themselves as parallel rods on an 

 asterless mitotic figure. Each of these double chromosomes shows its double 

 nature by a transverse projection in the middle, so that it has the form of a 

 cross with the horizontal arms very short. The whole group of chromosomes 

 then condenses till it makes an evenly stained mass which is more or less 

 oval in shape. This is the last stage seen before the egg is laid, and evidently 

 corresponds with the small evenly-stained nucleus at the edge of the 

 egg found in eggs of Neuroterus preserved immediately after deposition. It 

 is extremely like Pigs. 1 and 2 of Plate 17 in Part II of this series.* 



Further, P. Martinf finds the same phenomena in Ageniaspis, followed by a 

 maturation' division apparently of the type described in Neuroterus. 



It seems very probable, therefore, that the " top-shaped '• nucleus described 

 in Neuroterus is a stage in the disentangling of these chromosomes from the 

 compact condition, and that the separation of the inner group from the outer 

 in the first division corresponds with a separation of the members of the 

 end-to-end pairs from one another, while the second division would be, as 

 suggested above, a longitudinal splitting of single chromosomes. The 

 peculiarities of the first division may he supposed to arise from the fact that 

 the double chromosomes prepare for division some time before the egg is laid, 

 then become closely pressed together into a compact mass, and after the egg- 

 is laid separate from one another before they are completely disentangled 

 from the tight mass into which they have been concentrated.] 



* 'Roy. Soc. Proe.,' B, vol. 83 (1911). 



t ' Zeit. Wiss. Zool.,' vol. 110, p. 419 (1914). 



