PROTOPLASM 5 



(Fig. 5). Prolongations from the surrounding capsule-cells have been 

 described as entering these canals and as performing, together with the 

 lymph, a nutritive function. Hence the network of canals has been called 

 trophospongium. But it has not been shown conclusively that these 

 canals open to the exterior of the cell. They may be similar to the closed 

 networks or "reticular apparatus" lying wholly within the protoplasm, 

 shown in Fig. 6. Such networks have been described in nerve cells, carti- 

 lage cells and gland cells. The network is said to be of a thick fluid con- 

 sistency. In certain gland cells there are canals within the protoplasm, 

 which convey the secretion to the free surface of the cell. These may be 

 simple, branched, or arranged in a network. Like the other forms of 

 intracellular canals, they can be studied only in special preparations. 



5. Inclusions. Various foreign bodies, such as other cells or bacteria, 

 which may have been ingested by the protoplasm, are grouped as inclu- 

 sions. This term is applied also to crystalloid substances formed within 

 the protoplasm (Fig. 7), and to coarse masses of pigment granules which 

 appear extraneous. 



NUCLEUS. 



The nucleus (Latin, nucleus, "the kernel of a nut"; Greek, Kdpvov, "a 

 nut") is typically a well-defined round body, situated near the center of 

 the cell, appearing denser or more coarsely granular than the surrounding 

 protoplasm (Fig. i). There are characteristic variations in the shapes of 

 nuclei, in their position within the cells and in their structure. 



Ordinarily the karyoplasm, or nuclear substance, is sharply marked off 

 from the cytoplasm by the nuclear membrane. Sometimes, in preserved 

 tissues, the cytoplasm has shrunken away from the nuclear membrane, 

 so as to leave a narrow space partially encircling it; and in certain living 

 cells, the nucleus migrates through cytoplasm, as if it were an independent 

 body. But there are phases of cell-development in which the nuclear 

 membrane disappears and no line can be drawn between karyoplasm and 

 cytoplasm. At all times they have a common structural basis. The 

 ground substance of the nucleus, corresponding with the hyaloplasm, is 

 the nuclear sap; and it contains, for spongioplasm, a meshwork of delicate 

 linin fibrils. These help to form the nuclear membrane, in which they 

 terminate. The nuclear membrane, nuclear sap, and linin reticulum do 

 not stain deeply, and are therefore grouped together as the achromatic 

 constituents of the nucleus. 



The principal chromatic constituent of the nucleus is known as chroma- 

 tin. It stains deeply, since it contains a large amount of nucleic acid, 

 which has a marked affinity for basic stains. Chromatin occurs in the 

 form of granules, which are bound together in strands or masses by the 



