100 



Mr. L. Doncaster. 



[Sept. 21, 



the first segmentation mitosis. Minute centrosomes are visible at the 

 poles of the spindle, but I have not observed their origin. Sometimes at 

 least the chromosomes in the equatorial plate of this first division are seen 

 ito be in two groups side by side, so that the complete mingling cannot 

 take place till the first segmentation nuclei are reconstituted (fig. 35). 



In the later segmentation mitoses the chromosomes are elongated, and 

 appear to be about 18 to 20 in number; and in the nuclei just before 

 mitosis a similar number of chromatin bands may be counted (figs. 36 to 38). 



Maturatio7i and Segmentation of the Egcj of the Spring Generation. 



My observations on the eggs of the spring generation are not yet complete, 

 and cannot be finished until fresh material has been obtained. I give here 

 a preliminary account of the results, some of which cannot be regarded as 

 established with certainty, and leave the full description until the second 

 • part of this paper is published. 



The eggs of the spring generation are parthenogenetic ; they are laid 

 in the developing buds of the oak in April, and as has been mentioned 

 above, those laid by some females develop into males, those laid by others to 

 females. 



When the maturation divisions have been found they resemble the early 

 stages of the maturation in the summer eggs, but I have never seen the 

 division of the inner chromosome group, and am inclined to believe that it 

 may sink in and become the egg-nucleus without further division, so that 

 only one true maturation division takes place (Plate 3, figs. 39 to 43). 



In other eggs I have been unable to find any trace of a maturation 

 division, and in the later stages, during segmentation, no polar clironu)somes 

 are to be fouiul in such eggs, although they are always to be found in the 

 summer generation at the same stage. It seems probable, therefore, that 

 there are two kinds of eggs, of which one undergoes a maturation division 

 and the other does not, and tliis is confirmed by the study of the 

 segmentation mitoses. In tlie eggs laid by the majority of spring females, 

 20 chronjosomes are found in the segmentation mitoses (Plate 3, figs. 44 a, h ; 

 45, 46 a, b, c), but in the eggs laid by one female the division-figures all contain 

 10 or about 10 chromosomes (figs. 47, 48 a, h,c), and in some of these eggs 

 a double groii]) oi ])()lar chromosomes is clearly recognisable at the edge of 

 tlie egg in the place where the maturation mitoses have been found at an 

 earlier stage. Most of my material was collected before I discovered that 

 some individuals lay eggs whicli yield males, others wliich yield females, so 

 that I have only one series of eggs laid by one I'cmalo which show 

 the reduced number in the segmentation divisions. l>ut as far as they 



