Fig. 38.—Formation and conjugation of the pronuclei in Ascaris megalocephala. (From Quain’s Anatomy, after E. von Beneden.) f, female pronucleus; m, male pronucleus; p, one of the polar bodies.
I. The second polar body has just been extruded; both male and female pronuclei contain two chromatin particles; those of the male pronucleus are becoming transformed into a skein. II. The chromatin in both pronuclei now forms into a skein.
II a. The skeins are more distinct. Two attraction (or protoplasmic) spheres, each with a central particle united with a small spindle of achromatic fibres, have made their appearance in the general substance of the egg close to the mutually approaching pronuclei. The male pronucleus has the remains of the body of the spermatozoön adhering to it.
III. Only the female pronucleus is shown in this figure. The skein is contracted and thickened. The attraction-spheres are near one side of the ovum, and are connected with its periphery by a cone of fibres forming a polar circle, p.c.; e.c., equatorial circle.
III a. The pronuclei have come into contact, and the spindle-system is now arranged across their common axis.
IV. Contraction of the skein, and formation of two U-or V-shaped chromatin fibres in each pronucleus.
V. The V-shaped chromatin filaments are now quite distinct: the male and female pronuclei are in close contact.
VI., VII. The V-shaped filaments are splitting longitudinally; their structure of fine granules of chromatin is apparent in VII., which is more highly magnified. The conjugation of the pronuclei is apparently complete in VII. The attraction-spheres and achromatic spindle, although present, are not depicted in IV., V., VI., and VII.