MATURATION OF THE EGG 



[CH. 



in their equatorial plates. The two nuclei or chromosome 

 groups then divide again, giving rise to four groups of 

 chromosomes, which usually all form themselves into vesi- 

 cular nuclei. Of these the two outer, at the poles of the 

 outer spindle, are the halves of the first polar nucleus, and 

 the other two are the second polar nucleus and the mature 

 egg-nucleus. In some insects the polar chromosomes never 



FIG. 10. Early development of the egg of Platygaster. 



i 5. Platygaster dryomyiae, after SILVESTRI (1916). 6. P. ornatus slightly 

 modified from MARCHAL (1906). 



1. Three polar nuclei above, egg-nucleus and sperm-head below. 



2. The two inner polar nuclei have fused. 



3. The egg and sperm nuclei have fused. 



4. The protoplasm surrounding the zygote nucleus is separated as a 

 distinct cell ; the fused polar nuclei are giving rise to the/'paranucleus " 

 of the trophamnion. 



5. First segmentation division. 



6. Blastoderm stage ; paranucleus giving rise to nuclei of trophamnion. 

 See also Fig. 20, p. 181. 



form vesicular nuclei, but remain as groups of chromosomes 

 which gradually disintegrate. Not infrequently the second 

 polar nucleus conjugates with the inner half of the first, or 

 in some cases all three polar nuclei unite. With very rare 

 exceptions, however, the polar nuclei, whether united or 

 separate, disintegrate and disappear; the only known ex- 

 ception is in certain parasitic Hymenoptera (Chalcididae, 

 Proctotrypidae), especially in those in which polyembryony 



