NUCLEOLAR STRUCTURES OF THE CELL. 173 



o 



somewhat flattened epithelial cells, in the nuclei of which only 

 true nucleoli are present. But in the hypodermal cells of the 

 larval extremities both nucleoli and chromatin nucleoli occur 

 within the same nucleus ; and at the place where the extremity 

 joins the body proper the hypodermal nuclei offer a perfect 

 succession of gradations, showing all stages in the formation 

 of the chromatin nucleolus. The same gradations of nuclear 

 structure are also to be found where the hypodermis is invagi- 

 nated to form the stomodaeum and proctodaeum. 



At the proximal end of a larval extremity the nuclei contain, 

 besides the chromatin reticulum, one or two true nucleoli. As 

 we proceed, distad nuclei are found, which, in addition to the 

 true nucleoli, contain each one small granule, which stains ex- 

 actly like the chromatin, and for the designation of which the 

 name "chromatin nucleolus " is proposed. At the time when it 

 may first be clearly distinguished it is simply a small granule, 

 a little larger than the nodal points of the chromatin network. 

 While it is difficult to be positive of the conclusion, it would 

 appear to be a portion of this network, separated from the rest 

 of the chromatin. It is at the start usually in contact with a 

 true nucleolus, so that double nucleoli are thus produced. Now, 

 as the succession of stages is studied, it is found that the chro- 

 matin nucleolus steadily increases in size, while the true nucle- 

 olus attached to it either remains constant in volume or grows 

 much more slowly. A vacuolar substance appears within the 

 chromatin nucleolus and stains like the true nucleolus ; the nu- 

 cleus itself is increasing in size and gradually becoming amoe- 

 boid. Putting these facts together, — -though unfortunately 

 they can be mentioned here only briefly, — it appears that 

 to the true nucleolus new substance is continually being added 

 from without, while the chromatin nucleolus is absorbing mate- 

 rial from the true nucleolus. In no other way can be explained 

 the apposition of the two, accompanied by the rapid growth 

 of the chromatin nucleolus and the deposition of nucleolar-like 

 substance within the latter. Finally, in the large amoeboid 

 nuclei, which are found at the distal end of the extremity, 

 where these cells produce cuticular spines, a large chromatin 

 nucleolus is seen, but no longer any trace of true nucleoli. 



