Sex- Determination in the Gail-Fly, N. lenticularis. 199 



Series IV, 1 and 2. — Early stages seen in face, cut in two sections. In each the upper 

 figure represents the inner, the lower the outer portion. In IV, 2, there 

 are clearly about 10 threads drawn out towards the egg-centre ; in the 

 lower figure the dark spots are the cut ends which are continuous with the 

 threads in the upper figure. 

 IV, 3.— Inner portion of a similar figure, showing that the threads, at least near 



their bases, are double. 

 (IV, 1, 2, 3, should be compared with VII, 2, 3, 4, 5.) 



IV, 4. — Two figures from successive sections of a rather later stage in side 

 view, showing chromatin threads drawn out on both inner and outer side. 

 The staining of this slide (IV, 4) is unsatisfactory, and attempts to improve 

 it have proved unsuccessful. The figures are therefore to be taken as 

 sketches. 



IV, 5. — Telophase of maturation division. The egg- and sperm-nuclei are in 

 one section, but are represented nearer together than they actually are ; the 

 two inner polar groups and most of the outer are in the next section but 

 one. Some of the outermost polar chromosomes are hidden by the egg- 

 membrane. 



IV, 6. — Polar chromosomes after the completion of the maturation divisions 

 from three successive sections. Those of the inner group are clumped. 

 Series V, 1 and 2. — Two rather similar stages, showing complete separation of egg- 

 nucleus chromosomes from inner polar group before the outer group has 

 divided. In V, 1, the figure is reconstructed from two, in V, 2, from three, 

 successive sections, and the chromosomes are seen nearly in face. The later 

 eggs of this series show three groups of polar chromosomes, indicating that 

 the outer group divides. 

 Series VI, 1, 2, 3. — Three prophases somewhat intermediate between the "top-shaped :: 

 and " spherical " types. 

 VI, 4. — Later stage showing threads, double at their bases, drawn out towards 

 the egg-centre and shorter loops towards the outside. The lower figure 

 represents ends of threads seen in the next section ; the upper figure is 

 slightly spread for the sake of perspective. 

 VI, 5, 6. — Rather later stages : VI, 5, is cut in two sections ; VI, 6, is probably 



not well preserved, the inner and outer groups are in successive sections. 

 VI, 7. — -Double division-spindle, with a lagging chromosome in the outer group. 

 Series VII, 1. — Spherical nucleus before maturation. 



VII, 2. — Two sections of a later stage in face. The upper figure represents the 

 inner, the lower the outer, portion ; in the latter, the dark spots are the cut 

 ends of threads which appear in the upper figure. 

 VII, 3. — Oblique view of slightly later stage, showing double threads drawn 



inwards towards egg-centre. 

 VII, 4. — Similar stage cut in two sections in face. The upper figure is the 

 inner, and the dark dots in the lower are the cut ends of threads which pass 

 into the upper. 



VII, 5. — Similar figure, showing double nature of the threads towards their 

 bases. The 10 double threads forming an irregular outer ring are probably 

 chromosomes ; the three smaller pairs inside the ring are much fainter, and 

 are possibly spindle fibres. 



VII, 6. — Oblique view of similar stage. 



VII, 7. — Mitosis in side view, showing complete separation of the egg-nucleus 

 chromosomes from the inner polar group, while the outer group shows 



