54 THOS. H. MONTGOMERY jr., 



is reached they are usually hidden by the mantle fibres. Since, as 

 has been shown, the primary central spindle probably disappears, 

 this group of connecting fibres may be termed the secondary central 

 spindle. It would seem that the latter, unlike the primary one, is 

 a derivative of the nuclear linin. 



By the gradual tension of opposite pairs of mantle fibres, the 

 chromosomes are pulled into the plane of the equator (Figs. 160 — 

 169), where they eventually become arranged quite close together. 

 The fact of chief morphological importance in this process, is that it 

 results in the grouping of them with the line of their transverse con- 

 strictions in the equatorial plane (Figs. 160 — 169, 172—175). The 

 chromatin nucleolus also {n. 2, Figs. 161 — 169, 173 — 175) comes to 

 occupy a corresponding position, so that it too is destined to become 

 transversely halved. It is interesting that the latter structure has 

 attached to each of its ends two pairs of mantle fibres, which is 

 another point it shares in common with the chromosomes. Smaller 

 fragments of the original chromatin nucleolus sometimes also occur 

 in the region of the spindle, apposed to the pole or mantle fibres, 

 but they do not become divided nor constricted in mitosis (Figs. 163, 

 164). Yolk globules {YIc. Gl, Figs. 160, 161) are found not infrequently 

 at the periphery of the cell, but never in the chromosomal plate. 



In the completed monaster the 7 chromosomes lie close together 

 at the equator of the spindle, their transverse constrictions coinciding 

 with this plane, their long axes with the axis of the spindle (Figs. 169, 

 172 — 175). The transverse constriction, if not always situated at the 

 middle of a chromosome, at least always lies so that it demarcates 

 two portions of equal volume. In the great majority of cells, prob- 

 ably in every case, the chromatin nucleolus lies in the chromosomal 

 plate, and though frequently hidden on side views of the spindle, 

 may almost always be seen on pole view (n. 2, Figs. 176 — 189). So 

 regularly does it occupy this position , that I do not hesitate to con- 

 clude that in those cases where it cannot be seen, it is nevertheless 

 present and only obscured by the chromosomes. All the chromosomes 

 do not reach their definitive position simultaneously, one or two frequently 

 lagging behind the others (Figs. 161—168). A figure (Fig. 174) is 

 given of the solitary case found where the chromosomes were nearly 

 round in form , though in the smaller chromosomes of this abnormal 

 case the transverse constrictions could be seen. In all other cells 

 seen in the monaster stage, and I have seen more than a hundred 

 of them, all the chromosomes were plainly transversely constricted. 



