44 



THE ELEMENTS OF STRUCTURE. 



r, 2. — Structure of 

 the cell. (After 

 Carxoy.) 



Nucleus with chro- 

 matic coil, note proto- 

 plasmic reticulum. 



Within the nucleolus an " endo-nucleolus " has been dis- 

 covered. Though the nuclei of different cells differ in 



details, there is a fundamental sameness, both of structure 



and activity, throughout the world of cells. 



((-) TAe Cetitrosomes. — When a cell divides into two, the 



chromatin elements or chromosomes of the nucleus are also 



divided and separate to form the two 



daughter nuclei. In this separation 



extremely fine " archoplasmic " threads 



have been seen passing to the chromo- 

 somes from beside two minute bodies 



in the cytoplasm. These two bodies 



are called central corpuscles or centro- 



somata (Figs. 3 and 4); they seem to act 



like two centres of force. They also 



occur, in most cases singly, in resting 



cells, and it seems likely that they are 



constant parts of the cell, and that 



they arise from within the nucleus. 

 (d) The Cell Wall. — To the earlier 



histologists, who often spoke of cells 



as little bags or boxes, the wall seemed of much moment. 



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 distinguish- 

 able from the living matter itself. 

 J3ut the maigin 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 forma- 

 tion of a skin on cooling porridge. 

 In other cases, especially in cells 

 active, such as ova and encysted 



chr 



Fig. 3. — Fertilised ovum 

 of Ascaris. (After 

 BOVERI.) 



chr. Chromatin elements, 

 two from ovum nucleus 

 and two from sperm 

 nucleus ; cs. centrosoma 

 from^^ which "archoplas- 

 mic " thre.-ids radiate, 

 partly to the chromosomes. 



which are not very 



