IN THE OOCYTES OF THE GALL-FLY NEUROTERUS. 331 



being present. At first these chromosome pairs are scattered irregularly ; 

 but subsequently they take up a definite arrangement, which is in all respects 

 like the metaphase of a mitotic figure. They place themselves in a parallel 

 series around a central axis with the point of contact of each conjugant 

 indicated by a constriction in the equatorial plane. The chromosome com- 

 plex so formed then condenses to form the oval compact nucleus, which 

 is described by Doncaster in the eggs after they have been deposited 

 (fig. 1, 4, 3, 6). 



Hegner has given full details of similar phenomena, as already mentioned, 

 in Copidosoma ; and on most points his account agrees closely with that 

 given above for Neuroterus. In the former, <iccording to Hegner, " the 

 spireme becomes more and more open, and finally breaks up into thin 

 chromosomes of irregular shape. These chromosomes then become shorter 

 and thicker, and appear to unite near their ends. At first the pairs are 

 scattered about in the nucleus, but they soon straighten out and become 

 arranged in a parallel series with their points of union lying in the equator. 

 Spindle fibres could be seen, but apparently no centrosomes nor asters 

 are present. . . . The mitotic figure then passes through the stage of 

 condensation. . . . The chromosomes gradually get closer together and 

 become shorter and thicker. Where these ends meet at the equator a ridge 

 appears which causes the complex to resemble a Maltese cross. Soon the 

 spaces between the chromosomes are entirely obliterated and a homogeneous 

 mass of chromatin results." Elsewhere Hegner refers to the arrangement 

 of the chromosome pairs on a " Maturation spindle " in Apanteles, and in 

 Andricus, the Oak-knot Gall-fly closely allied to Neuroterus. In Neuroterus 

 the junction of the conjugating chromosomes is not indicated, as in these, by 

 a thickening, but a constriction. 



General Remarks. 



In a postscript to his final memoir on gametogenesis in Neuroterus, added 

 on receipt of Hegner's ' Protoplasmic Differentiation in the Oocytes of 

 certain Hymenoptera,' Doncaster expressed the belief that the top-shaped 

 nucleus, described by him in certain ova prior to maturation, is a stage in 

 the disentangling of a compact nucleus formed in some such manner as in 

 Copidosoma. This has been shown to be the case. The polar mitosis 

 described by Doncaster represents the type of figure that might be predicted 

 if the preparation for the homotypic split begins during the first division 

 and extends from the junction of the conjugants to their free distal ends. 

 The double threads drawn out in the formation of the first polar body from 

 the reticulum so formed would be equivalent to univalent chromosomes 

 precociously and incompletely split. The formation of the second body then 

 consists presumably in separating the two halves. 



