1909.] 



Gametogenesis of the Gall- Fly, etc. 



99 



yielding three groups. If this is the case, the innermost of the three groups 

 must be formed by a sorting out of certain chromosomes from the original 

 outer group, followed by a division of the remainder. It has been seen 

 that there is no evidence for more than one maturation division in the 

 spermatogenesis, but the existence of only one such division in the egg 

 does not seem consistent with the presence of the diploid number of 

 chromosomes in the oogonia. 



The further fate of the polar chromosomes seems to vaiy somewhat in 

 different eggs. In some, at the time of the conjugation of the male and 

 female pronuclei, they appear as a single large group surrounded by a field 

 clear of yolk granules, and with the individual chromosomes long and 

 thread-like. Very frequently there are two groups, as if the two inner 

 had amalgamated, and occasionally one finds three or four chromatin masses, 

 in these cases usually with the chromatin closely balled together. In 

 rather later stages, during the segmentation (about five to eight hours), the 

 polar chromosomes seem to shrivel into small irregular masses and then to 

 disappear completely. 



It may perhaps be suggested that the apparent abnormality of the whole 

 of the maturation processes of the egg is due to defective methods of 

 preservation, but I think it unlikely for the following reasons. In the 

 first place, at a rather later stage the conjugation of the pronuclei and 

 segmentation divisions are well preserved ; and secondly, Henking found 

 very similar abnormalities in Ehodites in eggs preserved by hot water and 

 by Flemming. It seems unlikely that three so difierent methods of preser- 

 vation should all give the same results if the phenomena were not genuine. 

 The division-figures in the cells of the leaf-tissue are also well fixed. 



While the maturation of the egg-nucleus is in progress the spermatozoon 

 is being converted into the male pronucleus. In eggs preserved within 

 half an hour after being laid it commonly appears as a long narrow rod, 

 straight or slightly curved, and often extending through two or even three 

 sections. It contracts to a small, oval, compact nucleus, which stains very 

 deeply, and then gradually swells, while chromatin bodies become visible 

 within it. At this stage the chromatin sometimes suggests a coiled thread, 

 but the staining is too dense to see whether this is so with certainty 

 (Plate 2, fig. 32). As it increases in size, the male pronucleus sinks into 

 the egg to meet the female pronucleus, which has now reached a similar 

 condition, and when they meet in the centre of the egg they are both very 

 large and vesicular (Plate 3, fig. 34). 



The chromatin masses become definite chromosomes, the nuclear membranes 

 disappear, and the two groups of chromosomes mingle and begin to form 



