NORMAL HISTOLOGY. 



refracting, and, when subjected to appropriate stains, takes on a 

 color differing from both nuclear fibrils and protoplasm ; it contains 

 a substance known as pyrenin. The nucleolus lies closely approxi- 

 mated to, but separated from, the nuclear fibrils, being especially 

 evident in such elements as ganglionic nerve-cells, or ova, where the 

 nucleolus appears with exceptional distinctness. Its disappearance 

 during the division of the nucleus, and its subsequent reappearance 

 within the newly-formed nuclei, lend weight to the supposition that 

 the nucleolus plays but a subordinate r6le in the life-history of the 

 cell ; its true value, however, has yet to be determined. 



In addition to the parts of the cell readily recognized, the presence 

 of a very small, constant, round, highly-refracting body the cen- 

 trosome has been established within the cytoplasm. This body 

 is so minute and inconspicuous that it is not easily distinguished ; 

 the fact that the centrosome is itself surrounded by a clear area, 



FIG. 4. 



FIG. 5. 



B 



A, cell from pancreas of salamander: n, 

 nucleus; /, paranucleus. B, sexual cell of 

 leech : , nucleus ; /, paranucleus ; c, centro- 

 some. (After Plainer.) 



U 



Segmenting ova of ascaris 

 megalocephala : n, nucleus ; 

 a, centrosome, surrounded 

 by attraction-sphere ; /, po- 

 lar body. (After Boveri.) 



named the attraction-sphere, will often aid in its detection. While 

 the centrosome has been shown to exist during the condition of rest, 

 it is especially in connection with the changes incident to the division 

 of the nucleus that it becomes most conspicuous, since its division 

 and subsequent migration often inaugurate the complex cycle of 

 nuclear changes. 



Certain investigators have described the occasional presence of an 

 irregularly spherical body, lying within the protoplasm in the vicinity 

 of the nucleus, to which the name accessory nucleus, or paranu- 

 cleus {Nebenkern of the Germans), has been applied. According 

 to Platner, the paranucleus is an extrusion of the nucleus, and is sub- 

 ject to great variation in size and appearance ; the existence, nature, 

 and function of this body are at present still uncertain, and need fur- 

 ther investigation. 



