28 



HISTOLOGY 



central bodies in the larger of these spaces, and these bodies have a 

 smoky and transparent appearance. The very smallest spaces contain 

 nothing at all that can be seen with the microscope. 



The nucleoli in nearly all cases contain vacuoles in their own sub- 

 stances, these vacuoles being filled with a substance which does not stain 

 deeply, but to the same degree as the bodies in the smaller karyoplasmic 

 spaces described above. These nucleolar vacuoles vary in size and 

 number; they increase in both as the cell grows older. The clear fluid 

 or substance which fills all the rest of the nucleus is a decidedly visible 

 body in the cell we are studying. Although sufficiently clear and free 

 from staining power to permit of our seeing all the other organs clearly, 

 yet it is distinctly visible in cells fixed in Flemming's fluid. Its density 

 or consistency is not known by any visible feature except perhaps the 

 fact that by coagulation it supports some of the parts and organs of the 

 nucleus. All the substances described are surrounded by the nuclear 

 membrane, which forms a complete covering to the entire nucleus. This 

 membrane has an appreciable thickness and is homogeneous in struc- 

 ture. It has considerable staining power, more than the linin and less 

 than the nucleoli. 



The first evidences of an approaching division are seen best in the 

 chromatin. Its particles appear blacker and larger, and a close inspec- 

 tion shows them to be more irreg- 

 ular in outline and to have ar- 

 ranged themselves in incomplete 

 rows in various parts of the kary- 

 oplasm, thus altering the granular 

 structure to a thread-like appear- 

 ance. A slight enlargement of the 

 nuclei, the appearance of larger 

 vacuoles and more of them in the 

 nucleolus, which becomes irregu- 

 lar in outline, and the growth and 

 complete alignment of the chro- 

 matin particles into threads now 

 cause the stage to become easily 

 identified and found among the 

 cells (Fig. 26). The chromatin at 



FIG. 26. Hyacinth root -tip cell showing first 

 signs of an approaching mitotic division. 

 Chromatin gathering in skein or spireme. 



this stage has formed a complete and apparently continuous thread, 

 called the spireme, in the meshes of which the smaller and much dis- 

 torted nucleolus is still seen. This thread is not only complete but its 

 ends are joined, forming a Gordian knot which is usually broken or 

 disturbed by the process of section cutting. It is of considerable 

 length. 



