CELL DLVISION. 45 



Protozoa, a more definite sheath is formed around the 



cell substance. Again, animal 

 cells may die off superficially 

 into a "cuticle," sometimes of 

 known chemical composition, as, 

 for instance, the chitin formed 

 by the ectoderm cells in Insects, 

 Crustaceans, and other Arthro- 

 pods. 



In animals, as well as in plants, 

 adjacent cells are often linked by 



„ „. f ,, intercellular bridsres of livins 



t: IG. 4. — Diagram of cell 00 



division. (After Boveri.) matter. 



, ^, . Cell Division. — Though the 



cliT. Chromosomes forming an j- - • r ii i 1 - i n 



equntorial plate ; M. centrosoma. dlVlSlOH Ot CCUs, by whlch all 



growth is affected, is a subject 

 with which the physiologist is as much concerned as the 

 morphologist, it will be convenient to discuss it here. The 

 following facts are most important. 



(i) We know that there is a striking unity in all cases, and 

 that the nucleus plays an essential part in the process. In 

 most cases the dividing nucleus passes through a series of 

 complex changes known as karyokinesis or mitosis, and 

 these are much the same everywhere, though different kinds 

 of cells have their specific peculiarities. Occasionally, how- 

 ever, both in Protozoa and Metazoa the nucleus divides by 

 simple constriction (direct or amitotic division). 



(2) The eventful changes of karyokinesis are as follows; — 



[a] The resti}ig stage of the nucleus shows a network or complete 

 coil of filaments (chromatin elements) (Fig, 2). 



(/') First Stage. — As division begins, the membrane separating 

 the nucleus from the cell substance disappears, and the 

 chromatin elements are seen as a tangled or broken coil 

 (Fig. S, !)• 



[c] Astroid-stage. — The chromatin elements bend into looped 

 pieces, which are disposed in a star, the free ends of the 

 U-shaped loops being directed outwards. Meanwhile, a 

 centrosome has appeared and divided into two separating 

 halves, between which a spindle of fine achromatin threads 

 is formed. This seems to form (at least part of) what is 

 called the nuclear spindle. The centrosomes separate until 

 one lies at each pole of the cell, surrounded by radiating 

 "archoplasmic " threads, which become attached to the 

 chromosomes (Fig. 5, 2). 



