46 THE ELEMENTS OF STRUCTURE. 



between the living matter of the nucleus and that of the 

 cytoplasm. Cytoplasm and nucleoplasm form a " cell firm," 

 potent in their co-operation. In many cells it has been 

 shown that fragments or extensions of the nucleus pass into 

 the cytoplasm, forming what is called a " chromidial appar- 

 atus," which seems to be of much functional importance. 



The nucleus often lies within a little 

 nest in the midst of the cell substance, 

 but it may shift its position from one 

 part of. the cell to another. It has a 

 definite margin, but this may be lost, 

 e.g. before cell division begins. Inter- 

 nally, it is anything but homogeneous 

 (see Fig. 22) ; at any rate, homogeneous 

 nuclei are rare. Twisted strands or 



FIG. 22. Structure tubes of " linin " bear a more stainable 



of the cell. After material called " chromatin," and when 

 Carn y- the cell is preparing to divide the 



N, Nucleus with chro- strands assume the form of a definite 



.matin coil; note pro- 

 toplasmic reticulum. number of separable rods or loops or 



granules, the "chromosomes." Sur- 

 rounding the linin and chromatin is the nuclear sap. 



Sometimes a linin thread shows a row of minute chromatin bodies 

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

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

 vehicles of the heritable qualities. He has made a hypothetical scheme, 

 according to which the chromosomes or idants are built up of ids, and 

 the ids of determinants, and the determinants of biophors. 



Many nuclei also contain little round bodies or nucleoli, 

 or sometimes a single nucleolus. The term is applied 

 somewhat vaguely to little aggregations 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 noted that when an 

 animal cell divides, these bodies play an important part. 

 Trie chromatin elements of the nucleus are divided, and 

 separate to form the two daughter nuclei. In this separa- 

 tion extremely fine " archoplasmic " threads 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 



