CELL-DIVISION. 43 



as Starch, and is a product, not a part, of the protoplasm, 

 though some protoplasm may be intimately associated with 

 it as long as its growth continues. In animal cells there is 

 rarely a very distinct wall chemically distinguishable from the 

 living matter itself. But the margin is often different from 

 the interior, and a slight wall may be formed by a superficial 

 compacting of the threads of the cell-network, or by a physical 

 alteration of the cell-substance, comparable to the formation 

 of a skin on cooling porridge. In other cases, especially 

 in cells which are not very active, such as ova and encysted 

 Protozoa, a more definite sheath is formed around the 

 cell-substance. Thirdly, animal cells may form a superficial 

 " cuticle," sometimes of known chemical composition, wit- 

 ness the chitin formed by the outer-layer cells in Insects, 

 Crustaceans, and other Arthropods. 



In animals, as well as in plants, adjacent cells are some 

 times linked by inter-cellular bridges of living matter. 



Cell-Division. — Though the division of cells, by which all 

 growth is affected, is a subject with which the physiologist 

 is as much concerned as the morphologist, it will be con- 

 venient 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 pro- 

 cess. It passes through a series of changes known as karyo- 

 kinesis, and these are much the same everywhere. But 

 it also seems true that a simpler mode of division (sometimes 

 called " direct ") occurs in some Protozoa and in some cells 

 of higher animals. 



(2) The eventful changes of the dividing nucleus are in 

 general terms as follows : — 



{a) The resting stage of the nucleus shows a network 

 of filaments (chromatin threads). 



(b) Coil Stage. — As division begins, the membrane 



separating the nucleus from the cell-substance dis- 

 appears, and the chromatin threads are seen as an 

 irregular coil like a small ball of twine. 



(c) Aster-stage. — The threads of the coil break into 



looped pieces which are disposed in a star, the 

 free ends of the U-shaped loops being directed out- 

 wards. Meanwhile, from two " central corpuscles," 



