CELLS. 



45 



chr. 



Fig. 20. — Fertilised ovum of 

 Ascaris. — After Boveri. 



chr., Chromatin elements, two 

 from ovum nucleus and two 

 from sperm nucleus ; cs. , 

 centrosoma from which 

 "archoplasmic" threads 

 radiate, partly to the chromo- 

 somes. 



In plant cells there is usually a very distinct wall, consisting 

 of cellulose. This is a product, not a part, of the pro- 

 toplasm, though some protoplasm may be intimately 

 associated with it as long as 

 its growth continues. In animal 

 cells there is rarely a very dis- 

 tinct wall chemically distinguish- 

 able from the living matter itself. 

 But the margin is often different 

 from the interior, and a slight 

 wall may be formed by a super- 

 ficial 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 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 as 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 karyo- 

 kinesis or mitosis, and these are 

 much the same everywhere, 

 of cells have their specific peculi- 

 however, both in Protozoa and 



chr- 



FlG. 21. — Diagram of cell 

 division. — After Boveri. 



chr.. Chromosomes forming 

 an equatorial plate ; cs., 

 centrosoma. 



though different kinds 



arities. Occasionally, 



Metazoa, the nucleus divides by simple constriction (direct 



or amitotic division). 



