NORMAL HISTOLOGY. 



present, little is definitely established. The nucleolus is highly 

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

 color differing from both nucleus and protoplasm, suggesting, at 

 least, a distinct chemical condition. This body lies closely approxi- 

 mated to, but separated from, the nuclear fibrils, being an indepen- 

 dent member of the cell; this fact is especially evident in such ele- 

 ments 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 role in the life-history of the cell; its true value, 

 however, has yet to be determined. 



In addition to the parts of the cell generally recognized, recent 

 investigators have described the occasional presence of an irregularly 

 spherical body, lying within the protoplasm in the vicinity of the 



FIG. 4. 



FIG. 5. 



B 



A, cell from pancreas of salamander: , 

 nucleus ; /, paranucleus. B, sexual cell of 

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

 some. (After Plainer.) 



ft 



Segmenting ova ofascaris 

 megalocephala : n, nucleus ; 

 a, centrosome, surrounded 

 by attraction-sphere ; /, po- 

 lar body. (After Boveri.) 



nucleus, to which the name accessory nucleus, or paranucleus 

 (Nebenkern of the Germans), has been applied. According to Plat- 

 ner, the paranucleus is an extrusion of the nucleus, and is subject to 

 great variation in size and appearance; the nature and function of 

 this body are at present still obscure, and need further investigation. 

 Likewise, the presence of a very small, round, highly-refracting 

 body the centrosome, or pole-corpuscle has been established in 

 sexual cells, and also in many other elements. The centrosome is 

 itself surrounded by an area named the attraction-sphere. While 

 these bodies have 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 their most conspicuous features have been ob- 

 served; much, however, remains to be determined regarding these 

 constituents of the cell. 



