18 



THE ANIMAL CELL. 



[CH. II. 



iu which case a double star (diaster) results (fig. 18, G, H, and i). 

 After remaining almost unchanged for some time, the V-shaped 

 fibres being first rearranged in the centre, side by side (angle out- 

 wards), split longitudinally and separate into two bundles which 

 gradually take up a position at either pole. From these groups of 

 fibrils the two nuclei of the new cells are formed (daughter nuclei) 

 (fig. 1 8, K), and the changes they pass through before reaching 

 the resting condition are exactly those through which the original 

 nucleus (mother nucleus) has gone, but in a reverse order, viz., 

 the star, the rosette, and the convolution. During or shortly 

 after the formation of the daughter nuclei the cell itself becomes 

 constricted and then divides in a line about midway between them. 



The changes as described are those which are most obvious : 

 but they take little account of the formation of the spindle seen in 

 fig. 1 8, F, nor of the part played by the attraction sphere (see p. 1 2). 



The work of Waldeyer, Rabl, and others has shown that a 

 more exact description is the following. 



The process may be divided into the following stages : 



i. The non-dividing nucleus (fig. 19). 



Node of network 



Node of network. 



-- Nuclear membrane. 

 ... Nucleolus. 



Nuclear matrix. 

 ... Nuclear network. 



Fig. 19. The resting nucleus. (Waldeyer.) 



2. The spirem or skein stage : the nucleoli dissolve, the 

 secondary fibres disappear, and the primary loops running 

 from polar to anti-polar regions remain (figs. 8, 20). 



In some cells there is at first one long, much twisted thread, which 

 subsequently breaks up into segments. 



3. Each loop becomes less convoluted and splits longitudinally 

 into two sister threads, and the achromatic spindle appears 

 (fig. 21, A and B). 



4. The eqiiatorial stage ; monaster. The nucleus has now two 

 poles, those of the spindle ; and at each pole there is a polar 

 corpuscle or centrosome. The division of the centrosome 

 of the original cell, and then of the attraction sphere into 

 two, usually precedes the commencement of changes in the 



