CELLS. 



47 



chr 



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 intim- 

 ately associated with it as long as 

 its growth continues. In animal 

 cells there is rarely a very dis- 

 tinct wall chemically distinguish- 



FiG. 23. Fertilised ovum of 

 Ascaris. After Boveri. 



able from the living matter itself. 



chr. , Chromatin elements, Uvo 

 from ovum nucleus and two 

 from sperm nucleus; cs., 

 centrosoma from which 

 " archoplasmic threads 



radiate, partly to the chromo- 

 somes. 



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. Again, 

 animal cells may secrete a 

 superficial " cuticle," e.g. the 

 chitin formed by the ectoderm 

 cells in Insects, Crustaceans, 

 and other Arthropods. 



In animals, as well a's in plants, 

 adjacent cells are often linked by 

 intercellular bridges of living 

 matter. 



In regard to cell division, the 



most important facts are the following :- -There is a striking 

 similarity in most cases, and the nucleus plays an essential 

 part in the process. The dividing nucleus usually passes 

 through a series of complex changes known as karyokinesis 



cs 



FIG. 24. Diagram of cell 

 division. After Boveri. 



chr., Chromosomes forming 

 an equatorial plate; cs., 

 centrosome. 



