44 



THE ELEMENTS OF STRUCTURE. 



FIG 2. Structure of 

 the cell. (After 

 CARNOY.) 



n. Nucleus with chro- 

 matic coil, note proto- 

 plasmic reticulum. 



Within the nucleolus an " endonucleolus " 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. 



(c) The Centrosomes. 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- 



F,G.3.-Fertilisedovum able from the living matter itself. 

 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 forma- 

 tion of a skin on cooling porridge. 

 In other cases, especially in cells 



which are not very active, such as ova and encysted 



of A scar is. 



BOVERl.) 



(After 



chr. Chromatin elements, 

 two from ovum nucleus 

 and two from sperm 

 nucleus ; cs. centrosoma 

 from which "archoplas- 

 mic " threads radiate, 

 partly to the chromosomes. 



