44 KARYOKINESIS. 



collects into the central area surrounding the centrosomes, and this area, thus delim- 

 ited from the surrounding plasm, is the sphere ; at the same time the spindle-fibres 

 again become plainly visible while a reticular or alveolar structure appears within 

 the spheres, fig. 58. 



In the late anaphase the spheres become much larger and are bounded by 

 a layer of microsomes from which fibers radiate. The interior of the spheres is 

 composed of a fine reticulum with nodal thickenings, and the whole sphere stains 

 much less densely than in earlier stages. Finally in the telophase the spheres reach 

 their greatest size and become filled with granules, the reticulum being scarcely 

 visible, or disappearing altogether (figs. 61, 68, 73). 



During the whole of the resting period the spheres persist, usually pressed close 

 to the cell-membrane, and as long as the centrosomes remain in them they preserve 

 a regular form (figs. 68, 69, 73, 74, 76). They are composed of coarse granules, 

 which stain deeply with plasma stains, and they are sharply bounded by one or 

 more layers of microsomes. As soon as the daughter centrosomes and central 

 spindle arise from the mother centrosome, they migrate out of the sphere and the 

 latter at once begins to lose its regular form. It becomes ragged in outline and 

 is finally flattened out to a thin layer of densely staining granules immediately 

 under the cell-membrane (figs. 63, 65, 70, 71, 72. 73, 74, 76). 1 



These granules, the remains of former spheres, can frequently be recognized 

 through two generations of cleavage cells; e. g., the spheres which appear in the 

 second cleavage (figs. 68-72) can still be recognized after the completion of the third 

 cleavage, located in the first quartette of micromeres (figs. 73 and 74, see also 

 figures of entire eggs in Plates V and VI). From the time when the daughter 

 centrosomes issue from the spheres the latter are degenerating structures, and 

 although their remains may persist for a surprisingly long time they ultimately 

 disintegrate and are apparently dissolved in the cytoplasm. 



To sum up the history of the spheres : we find that they arise around the cen- 

 trosomes at a very early period in the mitosis, in some cases within the mother 

 centrosome. With the disappearance of the nuclear membrane at the poles of the 

 spindle they are invaded by an interfilar substance ; they have no clearly marked 

 boundary. In the anaphase and telophase the spheres greatly enlarge, but their 

 growth is always proportional to the size of the cell in which they are found. They 

 are largest in the anaphase just before the chromosomal vesicles begin to form and 

 they probably contribute to the growth of the daughter nuclei. At first they have 

 a delicate radiating structure, this gives place to a homogeneous condition, and this 

 to an alveolar or reticular one; finally, in the rest stage they are granular. Their 

 fragments persist long after the daughter centrosomes have moved oat of them, and 

 they ultimately dissolve and disappear in the cytoplasm. 



1 In surface views of entire eggs the sphere may seem larger in the resting stage or early prophase 

 than in the telophase, e.g., figs. 81 and 82, 86 and 87, etc. ; this is due, as sections show, to a flattening 

 of the sphere against the cell-membrane and a spreading of the sphere substance through the influence 

 of the astral rays, and not to an actual increase in its volume (figs. 71, 72, 76). 



