of the Unfertilized Egg in the Satvfly. 475 



which may possibly be of significance, or may be due to the diffi- 

 culty of counting, as they are extremely small and much crowded. 



In N. ribesii I have been unable to find centrosomes in the 

 maturation mitoses, although in N. lacteus a body appears which 

 may be of this nature, but in the somatic mitoses some hours 

 later the centrosomes are conspicuous both in virgin and fertilized 

 eggs. 



From the two spindles described four nuclei arise, lying in the 

 polar protoplasm in a line perpendicular to the edge of the egg, 

 and at roughly equal distances from one another. They all 

 become reticular, with a well-marked membrane, and the two 

 outer ones are the daughter-nuclei of the first polar body, the 

 third is the second polar nucleus, and the innermost the egg- 

 nucleus (female pronucleus). The outermost soon becomes flattened 

 against the edge of the egg, and rapidly disappears. The second 

 and third move toward one another, meet, and become flattened 

 against each other ; their membranes disappear and the chromatin 

 reappears as chromosomes, which become mingled into a single 

 group*. No radiations are seen in the surrounding protoplasm. 

 Meanwhile the egg-nucleus has sunk deeper into the egg, leaving 

 the polar protoplasm and moving inwards and forwards among the 

 yolk granules. In all points therefore the fusion of the second 

 polar nucleus with the inner half of the first, and the travelling 

 away of the egg-nucleus, resembles the process described by 

 Petrunkewitsch in the virgin egg of the Honey Bee. 



The stage now described is reached about two hours after the 

 egg is laid, and the subsequent changes in the " conjugation- 

 nucleus," as it is called by Petrunkewitsch, during the next eight 

 hours are comparatively small. The chromosomes have formed a 

 compact group, containing from 14 to 16, the exact number being 

 still uncertain, but they usually soon begin to separate themselves 

 into two groups lying at varying but small distances apart, in the 

 polar protoplasm. At a rather later stage (3f hours) in one at 

 least of these two groups each chromosome sometimes appears to 

 be double, and still later (5^ hours) some preparations show twice 

 as many chromosomes in one group as in the other, or there may 

 be three groups each containing about eight. Beyond this no 

 change takes place up till the tenth hour, at which time the 

 blastoderm is beginning to form, and the egg becomes so full of 

 nuclei in various stages of division that I have not yet succeeded 

 in tracing the fate of the : ' copulation-nucleus " further. 



Returning to the egg-nucleus immediately after the second 

 maturation mitosis, we find that after travelling imvards and for- 

 wards through the yolk for some distance, it begins to divide, and 



* At about this stage in some sections an evenly and faintly stained round body, 

 rather smaller than a nucleus, is seen in the polar protoplasm. Its nature is at 

 present unknown to me. 



