46 THE ELEMENTS OF STRUCTURE. 



importance in connection with the respiration of the cell. 

 It is certain that there are complex actions and reactions 

 between the living matter of the nucleus and that of the 

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

 potent in their co-operation. 



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. Internally, 

 it is anything but homogeneous ; at any 

 rate, homogeneous nuclei are rare. 

 Usually there is a network of fine, 

 FIG. 22. Structure strongly stainable (chromatin) strands, 

 of the cell. After with less stainable (achromatin) sub- 

 Carnoy. stance in the meshes. In other cells, or 



N, Nucleus with chro- at another time in the same cell, the 



matin coil : note pro- i , , 



topiasmic reticuium. nucleus is seen to contain a coiled 



(chromatin) thread, or a definite num- 

 ber of chromatin bodies (chromosomes) (Fig. 22). 



Sometimes a chromatin thread appears to consist of a row of minute 

 bodies (microsomata) lying on a clear band (linin), 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 one or more little round bodies 

 or nucleoli, apparently of less importance. 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. These centrosomes are 

 therefore regarded as "division organs," or as "dynamic 

 centres." They also occur, in most cases singly, in resting 



