CELLS. 



47 



chr 



FlG. 23. Fertilised ovum of 

 Ascaris. After Boveri. 



chr., Chromatin elements, two 

 from ovum nucleus and two 

 from sperm nucleus ; cs., 

 centrosome from which 

 " archoplasmic " threads 



radiate, partly to the chromo- 

 somes. 



cells, and it seems likely that they are present in most 

 animal cells, at least in those which retain the power of 

 division. 



(d) As to the cell wall, it seemed 

 of much moment to the earlier 

 histologists, who often spoke of 

 cells as little bags or boxes. It 

 is, however, the least important 

 part of the cell. In plant cells 

 there is usually a very distinct 

 wall, consisting of cellulose. This 

 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 in- 

 terior, 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, especi- 

 ally in cells which are not very 

 active, such as ova and encysted 

 Protozoa, a more definite sheath 

 is formed around the cell sub- 

 stance. Again, animal cells may 

 secrete a superficial "cuticle," e.g. 

 the chitin formed by the ectoderm 

 cells in Insects, Crustaceans, and 

 FIG. 24. Diagram of cell other Arthropods. 



division. After Boveri. In animals, as well as in plants, 



chromosomes forming adjacent cells are often linked 

 CSm * by intercellular bridges of living 

 matter, which may be paths for 

 the passage of materials or of disturbances from cell to cell. 

 In many cases, e.g. of gelatinous tissue, a matrix arises out- 

 side of and between the cells, as an exoplasmic product. 

 In regard to cell division^ the most important facts are the 



an equatorial 

 centrosome. 



plate; 



