ZOOLOGY. 



non-nucleated part soon dies. Sometimes the nucleus divides in a 

 perfectly direct manner, just as a lump of putty or dough might be 

 divided into two by nipping it round the middle with a piece of 

 string ; but this is rare, and is characteristic of degenerating cells. 

 More usually a very complex series of changes takes place in 



Fig. 7. KARYOKINESIS. 



the chromatin-meshwork, of which the following is a general 

 description. The first change is said to be the division of the 

 centrosphere into two. The meshwork, which was irregularly 

 tangled, takes on the form of a simple thread folded in a series of 

 loops (the number of loops being apparently constant for all the 

 cells of a particular animal species, but varying in different species) 

 (fig. 7, B). 



The loops then become separated from one another, forming a 

 series of U- or V-shaped structures (chromosomes), which arrange 

 themselves in the centre of the nucleus in a radiating manner, the 

 free double ends pointing outwards (Star or Aster stage). At this 

 stage, the nuclear membrane disappears, the rest of the achromatin 

 mingling with the hyaloplasm ; in this mingled portion a fine 

 series of threads (fibrils) appear, radiating out from the two ccnlro- 

 spheres (which have moved to opposite sides of the chromatin- 

 rosette), and forming the spindle (fig. 7, c). Each chromosome 

 now splits into two, longitudinally, and the two halves, each still 

 V-shaped, separate from one another, and travel along the fibrils of 

 the spindle towards opposite centrospheres (fig. 7, D). Thus in place 

 of one radiating set of chromosomes we have two sets (Blaster 

 stage). From this point onwards we really have two nuclei, and the 

 changes undergone in each one are, in reverse order, the same 

 that the single nucleus went through at the beginning. 



It is impossible in our present state of knowledge to give any 

 adequate explanation of this complicated series of changes. But 

 the essential feature of the process is that nuclear division is not a 

 rough cutting in half, but a careful division of every part of the 

 chromatin of the nucleus into two equal parts. 



12. Differentiation of Cells. Animals which, like 

 amoeba, consist of one cell only (or of a few cells all equally 



