18 



CELL DIFFERENTIATION AND THE ELEMENTARY TISSUES 



stance which is rich in proteids, has but slight affinity for stains, and is supposed 

 to be fluid. 



The network is composed of linin or achromatin, a transparent unstainable 

 framework; and of chromatin, which stains deeply. It is supported by the 

 linin, and occurs sometimes in the form of granules, but usually as irregular 

 anastomosing threads, both thicker primary fibers and thinner connecting 

 branches. The threads often form thickened nodes, karyosomes or false 

 nucleoli, at their points of intersection. It is now quite generally believed that 



Cell membrane. 



Metaplasm gran- < 

 ules. 



Karyosome or net- 

 knob. 



Hyaloplasm. 

 Spongioplasm. 



Linin network. 

 Nucleoplasm. 



Attraction sphere. 



Centrosome. 



, Plastids. 



.-"- Chromatin network 

 Nuclear membrane. 



Nucleolus. 



Vacuole. 



FIG. 14. Diagram of a Typical Cell. (Bailey.) 



the chromatin occurs as short, rodlike, and highly refractive masses, which are 

 embedded in the linin in a regular series. 



The nucleoli, or plasmosomes, are spherical bodies which stain deeply, and 

 may either lie free in the nuclear matrix or be attached to the threads of the net- 

 work. 



The Centrosome and Attraction Sphere. In addition to the nucleus, 

 a minute spherical body called the centrosome is believed to be constantly 

 present in animal cells, though sometimes too small to be demonstrated. 

 The centrosome is smaller than the nucleus, close to which it lies, and exerts a 

 peculiar attraction for the protoplasmic filaments and granules in its vicinity, 

 so that it is surrounded by a zone of fine radiating fibrils, forming the attraction 

 sphere or archoplasm. Some authorities assert that the centrosome lies within 

 the nucleus in the resting state, and passes into the cell proper only in the earlier 

 stages of cell division. The attraction sphere is most distinctly seen in cells 

 about to divide. It plays an important role in nuclear division, but it is 

 doubted if it gives the initial impulse to the process. 



Cell Multiplication. Cells increase in number by a process known 

 as cell division, of which the first act is nuclear division. In fact the nucleus is 

 the center of control of the cell-mass in the process of division. Cell multi- 



