STRUCTURE OF PROTOPLASM 9. 



the mass of protoplasm into individual cellular units, there is formed what 

 is known as a syncytium. All syncytial structures are multi-nuclear, other- 

 wise their protoplasm is not unlike that of the cell as a unit. 



The fuller detail of cellular types and cell structure is given a little later 

 in Chapter II. However, for the purpose of reference to this point, one may 

 here call attention to the fact that the principal differentiations of the proto- 

 plasm of the cell are the nuclei and the cytoplasm. The cytoplasm is differ- 

 entiated further into two substances, spongioplasm and hyaloplasm. The 

 spongioplasm or reticulum forms a fine network, increases in relative amount 

 as the cell grows older, and has an affinity for staining reagents. The 

 hyaloplasm is less refractile, elastic, or extensile, and has little or no affinity 

 for stains; it predominates in young cells, is thought to be fluid, and fills the 

 interspaces of the reticulum. The nodal points of the reticulum, with the 

 granular microsomes, found in the protoplasm, cause the granular appear- 

 ance. The arrangement of the reticulum varies considerably in different 

 cells, and even in different parts of the same cell. 



In some cells, particularly in plants, but also in some animal cells, there 

 is a tendency toward the formation of a firmer external envelope, constituting 

 in vegetable cells a membrane distinct from the more central and more fluid 

 part of the protoplasm. In such cases the reticulum at the periphery of 

 the cell is made up of very fine meshes. The membrane when formed is 

 usually pierced with pores by which fluid may pass in, or through which 

 protrusion of the protoplasmic filaments forming the cell's connection with 

 other cells surrounding it may take place. 



FIG. 9. A: The Colorless Blood Corpuscle, Showing the Intracellular Network, and 

 two nuclei with intranuclear network. B: Colored blood corpuscle of newt showing the 

 intracellular network of fibrils. Also oval nucleus composed of limiting membrane and 

 fine intranuclear network of fibrils. X 800. (Klein and Noble Smith.) 



All protoplasm at some period of its existence possesses one or more neclei. 

 The origin of a nucleus in a cell is the first sign of the differentiation of proto- 

 plasm. The existence of nuclei was first pointed out in the year 1833 by 

 Robert Brown, who observed them in vegetable cells. They are either small 

 transparent vesicular bodies containing one or more smaller particles called 

 nucleoli, and always when in the resting condition bounded by a well-defined 



