V 



TELOPHASES. RECONSTRUCTION OF THE DAUGHTER-NUCLEI. CLEAVAGE. 29 



VII. TELOPHASES. RECONSTRUCTION OF THE DAUGHTER-NUCLEI. CLEAVAGE. 



The closing phases of karyokinesis, which differ considerably in different kinds of cells, are very beautifully shown in 

 Toxopncustcs. The history of the chromatic and achromatic structures may be separately considered. 



A. The Chromatin. — Immediately after the stage just described, each chromosome is converted into a small sac or 

 vesicle (Te.\t-fig. XVII. B), a clear space appearing in the interior, while the chromatin forms an irregular wall about it. 

 The vesicles then rapidly fuse together, their number becoming progressively reduced until only two or three larger 

 nuclear vesicles are left on either side (Te.\t-fig. XVIII.). The egg now divides into two, and as it does so, the remaining 

 nuclear vesicles unite on either side to form a single daughter-nucleus (Text-fig. XIX.). 



The formation of the chromatic vesicles seems to be of wide occurrence, especially in the early stages of develop- 

 ment. It is, perhaps, comparable with the secondary splitting of the daughter-chromosomes, observed by Flemming in 

 the heterotypical karyokinesis of the testi.s-cells in the Salamander, and by Van Beneden and Herla in Ascaris. 



B. The Achromatic Structures. — As the chromosomal vesicles are formed, they pass into the outer portion of the 

 centrosphere, which now rapidly changes its character, its outline becoming indistinct, and its staining capacity changing. 

 It no longer stains clear red in the double-stain, but purple, and is filled with blue granules. As the vesicles fuse, and the 

 cell divides, the centrosphere nearly disappears, leaving, however, an irregular and shrunken remnant, which, in some cases 

 at any rate, extends completely around the daughter-nuclei. The aster, as a whole, however, persists. 



As the cell divides, the mid-body (cf. Text-fig. XVII.) lies exactly in the line of division as a somewhat lens-shaped 

 mass of blue granules. Upon completion of the division it can no longer be found ; and, moreover, the remaining portions 

 of the spindle-fibres completely disappear. The aster is then left (Text-fig. XVIII.) as a horseshoe-shaped body, lying 

 beside the nucleus in each dauohter-cell. Soon afterwards it divides into two to form the asters for the succeedinsf 



o o 



cleavage. 



Plate IX. Phototype 34. 



The Chroinosojual Vesicles. 



The central portion of the spindle has broken up into granules (aggregated to form the mid-body in the following 

 stage), the peripheral portion is indistinguishable from the aster, of which it forms a part. Within each aster may be seen 

 a group of minute oval vesicles derived from the daughter-chromosomes of the last stage. The centrospheres are 

 very large, but ill-defined (Text-fig. XVII. B). 



Plate IX. Phototype 35. 

 Cleavacre. 



o 



The section passes horizontally through the egg at the moment of division. In the centre lies the mid-body, 

 darker and somewhat ill-defined. On either side of this is a group of 2-3 nuclear vesicles formed by the fusion of 



