54 



THE ELEMENTS OF STRUCTURE 



chemically (in fixation) it coagulates readily in a pattern 

 which is anything but homogeneous (see Fig. 26). Twisted 

 strands or tubes of " linin " bear a more stainable material 

 called " chromatin," and when the cell is preparing to 

 divide the strands assume the form of a definite number of 

 separable rods or loops or granules, the " chromosomes." 

 The number of chromosomes is in general constant for 

 each species of animals and plants. Surrounding the 

 linin and chromatin is the nuclear sap. 



Sometimes a linin thread shows a row of minute chromatin bodies 

 (microsotnata), like jewel-stones embedded on a belt. Weismann 

 suggested that the chromosomes or idants of the germ-cells are the 



vehicles of the heritable qualities or of some 

 of them ; and this view is generally accepted. 

 They carry the hereditary " factors " or 

 " genes," apparently in a linear arrangement. 



Many nuclei also contain little round 

 bodies or nucleoli, or sometimes a 

 single nucleolus. The term is applied 

 somewhat vaguely to little aggrega- 

 tions of chromatin, and more properly 

 to vacuole-like bodies, in which some 

 believe that the waste products of the 

 nucleus are collected. 



{c) As to the centrosomes, it may be 

 note protoplasmic notcd that whcu an animal cell divides, 

 SJJedb^'fixhit'S these bodies play an important part. 



The chromatin elements of the nucleus 

 are divided, and separate to form the two daughter nuclei. 

 In this separation extremely fine " archoplasmic " threads 

 appear to pass from the centrosomes to the chromosomes. 

 The centrosomes are therefore regarded as " division 

 organs," or as " dynamic centres." They also occur, in 

 most cases singly, in resting 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 



Fig. 26. — Structure of 

 the cell. — After 

 Carnoy. 



N ., Nucleus with chromatin 

 coil 



