THE STRUCTURE OF THE CELL 7 



During interphase, the centriole may often be distinguished 

 near the middle of the mass of cytoplasm situated at the opposite 

 pole of the cell from the nucleus. This body is too small for the 

 light-microscope to reveal any structure in it. Astral rays and one 

 end of the spindle were centred on it at the preceding division. 

 We have no proof that it always persists from one cell-division to 

 the next. Shortly before the next division two centrioles are 

 present, close to each other. 



The centriole is surrounded by a subspherical but rather 

 irregularly-shaped body. This was named Idiozom,^'^^ because it 

 forms a special envelope round the centriole (Greek idios^ 

 special ; zdma, envelope). The word may be written 'idiozome' in 

 English, but it is wrong to spell it with an s in place of a 2, though 

 this is often done.^^^ It is difficult to see in the untreated, living 

 cell, but in some cases it can be made evident by the use of vital 

 dyes (neutral red, dahlia). It appears structureless when examined 

 by the light-microscope, and no definite limiting membrane can 

 be seen. It is present continuously from one cell-division to the 

 next, whether the centriole persists or not. The chemical com- 

 position of the idiozome has not been worked out fully. It is not 

 so markedly different from the rest of the cytoplasm in chemical 

 composition as to be readily dyed in a contrasting colour. It has, 

 however, a somewhat greater affinity for the colouring agents 

 called 'acid' dyes (p. 90). If a suitably fixed testis is soaked in a 

 solution of osmium tetroxide, the latter is reduced at its surface to 

 produce a deposit of the black hydrated dioxide. The idiozome 

 corresponds with what some authors have called the centrosome, 

 but it is probably best to avoid this word, since it has been applied 

 also to the centriole. The functional significance of the idiozome 

 remains obscure. 



In many kinds of cells, especially blastomeres, there is a body 

 showing general resemblance to the idiozome, but the astral 

 rays of the preceding mitosis persist in continuity with it, so that 

 it appears spiky; it is commonly granular. This body in blasto- 

 meres was at first regarded as constituting the essential part of 

 the protoplasm: indeed, the name protoplasm was sometimes 

 restricted to the material of this particular object.*" The word 



