CELL WALL 



55 



Fig. 27. — Fertilised 

 ovum of Ascaris. — 

 After Boveri. 



chr., Chromosomes, two from 

 ovum nucleus and two from 

 sperm nucleus ; cs., centro- 

 some from which " archo- 

 plasmic " threads i^adiate, 

 partly to the chromosomes. 



part, of the protoplasm, though some protoplasm may be 

 intimately associated with it as long as its growth con- 

 tinues. 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 physical 

 alteration of the cell substance, com- 

 parable 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. 



The " plasma membrane," which fornis the outer 

 boundary of the protoplasm, is invisibly thin ; its exist- 

 ence and properties are deduced indirectly from experi- 

 ments on its impermeability to various substances. The 



" micro-dissection " experiments 

 of Chambers have made it more 

 real to us in the last few years, 

 and have emphasised its great im- 

 portance in the life of the cell. 



In animals, as well as in plants, 

 adjacent cells are often linked by 

 intercellular bridges of living 

 matter, which may be paths for 

 the passage of materials or of dis- 

 turbances from cell to cell. In 

 many cases,- e.g. of gelatinous 

 tissue, a matrix arises outside of 

 and between the cells, as an exo- 

 plasmic product. 

 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 



chr 



cs 



Fig. 28. — Diagram of cell 

 division. — After Boveri. 



chr., Chromosomes forming an 

 equatorial plate ; cs., centro- 

 some. 



