560 A. C. WALTON 



Meanwhile the female pronucleus (fig. 56) has begun to form 

 a similar linin network and soon (fig. 57) establishes a local- 

 ized center for the chromatic granules which result from the 

 breaking down of the chromosomes. 



In the male pronucleus the linin network assumes a wide- 

 meshed condition, and then (fig. 57) each strand becomes much 

 thicker. The chromatin at the same time migrates outward from 

 the central mass along each strand, and becomes so widely dis- 

 tributed that it loses the blue-black appearance characteristic 

 of massed chromatin when stained with iron-haematoxylin. The 

 development of the male pronucleus does not proceed beyond 

 this condition (fig. 58) until the female pronucleus has reached 

 the same stage. At this period it is not possible to distinguish 

 one type of male pronucleus from the other, nor either from the 

 female pronucleus. 



Up to this time the pronuclei have been widely separated, 

 but now they come to lie side by side. The centrosome, mean- 

 while, has divided and become surrounded by a mantle of slightly 

 denser alveolar cytoplasm. This method of formation of the 

 astrosphere has been shown for A. megalocephala by Erlanger 

 ('99), Retzius ('12), Vejdovsky ('12), and others; it does not 

 support the'archoplasm' theory of Boveri. The closely joined 

 centrosome 'couplet' occupies a position midway between the 

 two pronuclei, and when the latter approach each other its 

 two components move asunder (figs. 55 to 59) until they are as 

 far apart as the diameter of the pronucleus and about equally 

 distant from a line joining the center of the two nuclear 

 bodies. 



During the movement of the two pronuclei toward each 

 other the chromatin of each collects into numerous large, ir- 

 regular clumps at the junctions of the linin mesh work (fig. 59), 

 and the plasmosomes fuse into a few, larger, peripherally located 

 bodies. As the two pronuclei come to lie side by side (fig. 60), 

 the linin network begins to disappear, leaving the karj'-osomes 

 lying free in the karyoplasm. The karyosomes now have the 

 number characteristic of the chromosomes, that is there are 

 either twelve or eighteen for the male, and eighteen for the fe- 



