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 
Fic. 22, —Structure tubes of “linin” bear a more stainable 
of the cell.—After material called “chromatin,” and when 
Carnoy. the cell is preparing to divide the 
W, 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 zdazés are built up of zds, and 
the ids of determinants, and the determinants of dzophors. 
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. 
The 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 
