BIOLOGY OF PARACOPIDOSOMOPSIS. 6 1 



center is the ootid group. The fate of the polar bodies will be 

 considered later. The ootid group soon forms a small, condensed 

 female pronucleus, which approaches the male pronucleus with 

 which it eventually fuses. The pronuclei come together in the 

 lower half of the egg. Apparently for a considerable period they 

 lie close together before the actual fusion occurs. In Fig. 13 

 the two condensed pronuclei lie a short distance apart, with a 

 small aster and its tiny centrosome lying between them. This 

 case is interesting because it is the only one in which I have 

 observed either a centrosome or aster in the eggs of this 

 species. 



The pronuclei remain apart while they undergo expansion, 

 the female nucleus always being somewhat larger than the male 

 nucleus (Fig. 15). Upon coming in contact with each other, 

 the pronuclei fuse (Fig. 14) to form a single large conjugated, or 

 cleavage nucleus (Fig. 16). Many different stages of conjugating 

 nuclei have been observed in the preparations, one slide alone 

 showing more than a dozen cases. 



DISCUSSION. 



These observations show that in the egg of Paracopidosomopsis 

 two typical maturation divisions occur, resulting in reducing the 

 sixteen chromosomes of the primary oocyte to eight in the ootid. 

 It was shown in the second paper of the series that the sperm also 

 receives eight chromosomes. The fertilized egg should therefore 

 have sixteen chromosomes. A study of cleavage divisions shows 

 that this is the case. Fig. 17 is a polar view of a metaphase 

 plate of the first cleavage spindle. It has sixteen chromosomes. 

 In Fig. 1 8 a side view of an anaphase stage is shown. This is 

 one of the first four blastomeres undergoing division. There 

 are fifteen chromosomes lying toward each end of the spindle, 

 and the sixteenth, just completing its division, lies at the middle 

 of the spindle, on the right. 



A study of somatic cells shows that the diploid number of chro- 

 mosomes is also present in them. Thus the dividing cells of the 

 central nervous system of female larvae have sixteen chromosomes 

 (Figs. 3,4). On the other hand, dividing cells from the central 

 nervous system of male larvae have but eight chromosomes (Figs. 



