STRUCTURE OF PROTOPLASM 9 



8, the mesh work has an elongated radial arrangement from the nucleus; at 

 others, the mesh work is more evenly disposed, as in figure 9. At the junc- 

 tions of the fibrils there are usually slight enlargements or nodes. 



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

 is a tendency toward a 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. 



It is an exceedingly interesting question whether in cells the one part of 

 the protoplasm can exist without the other. Schafer summarizes the mat- 

 ter thus: "There are cells, and unicellular organisms both animal and vege- 



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.) 



table, in which no reticular structure can be made out, and these may be 

 formed of hyaloplasm alone. In that case, this must be looked upon as the 

 essential part of protoplasm. So far as ameboid phenomena are concerned 

 it is certainly so; but whether the chemical changes which occur in many 

 cells are effected by this or by spongioplasm is another matter." 



The Cell Nucleus. All cells at some period of their existence pos- 

 sess nuclei. The origin of a nucleus in a cell is the first trace of the differentia- 

 tion of protoplasm. 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, always when in the resting condition bounded by 

 a well-defined envelope. In their relation to the life of the cell they are 

 certainly hardly second in importance to the cytoplasm itself, and thus Beale is 

 fully justified in comprising both under the term " germinal matter." They 

 control the nutrition of the cell, and probably initiate the process of sub- 



