G 



INTRODUCTION PROTOPLASM CELL 



or granules (Fig. 5). The .si) ape of the nucleus is spherical; typically 

 it has a central location, but it frequently assumes eccentric positions. 

 It is physically denser and more elastic than the extranuclear protoplasm. 

 Its periphery simulates, or perhaps consists of, a membrane, the nuclear 

 irall. Whether as a membrane it be complete or reticulated, whether of 

 nuclear, cvioplasmic or composite origin, are undecided points. Recent 



investigations on the nuclear membrane indicate 

 that it is fenestrated; such conditions would per- 

 mit of an easy escape of nuclear material into the 

 cytoplasm. 



Nucleus. The protoplasm composing the nu- 

 cleus is known as nudeo plasm or karyoi>lasm; 

 that constituting the remainder of the cell, the 

 (yloi>!<tvi. The nuclear constituents include a more 

 fluid ground substance or nuclear sap (Tcaryo- 

 lijmiih : paralinin), throughout which extends a 

 delicate reticulum of linin threads (Fig. 4). 

 Upon these linin or achromatin threads are sup- 

 ported, more abundantly at the points of inter- 

 section of the mesh, granules (chromioles) and 

 masses (net-knots], of a substance, staining deeply 

 in the basic dyes, the chromatin. Spheroidal net- 

 knots are known as karyosomes. The linin is 

 said to be achromatic. Whether it is chemically 

 different from chromatin or simply more atten- 

 uated chromatin is disputed. The 'chromatic' gran- 

 ules themselves undergo changes in stainability : 

 on the basis of reaction to acid and basic dyes, 

 this substance is divided into oxy chromatin (lan- 

 tanin} and basichromatin. Limn and chromatin 

 are regarded by some as different phases in the 

 elaboration of the same substance. The nucleus in- 

 cludes, furthermore, usually one, frequently more, 

 nucleoli. These do not grade into the nuclear sap. like the nuclear 

 network, but are limited by a sharp line of demarcation. They may be 

 achromatic, when they are known as plasmosomes, or they may take on 

 chromatin, becoming cJiromatin nucleoli. It is uncertain whether the lat- 

 ter are identical in all cases with the karyosomes. The difference among 

 nucleoli is more probably one of degree of abundance of chromatin. The 

 nucleus is the metabolic organ of the cell; without a nucleus a cell may 



FIG. 5. A UNI- 

 CELLULAR FLAG- 

 ELLATE ANIMAL 

 (TETRAMITUS 

 CHILOMONAS). 



Showing the nu- 

 clear material dis- 

 tributed as granules 

 throughout the cell. 

 (Redrawn from 

 Calkins.) 



