32 



PRINCIPLES OF ANIMAL BIOLOGY 



examples of this condition. Such scattered granules are spoken of as a 

 distributed nucleus. In the red cells of human blood the nucleus is 

 actually wanting, although in their developmental stages they possess 

 nuclei which later disappear. 



The form of the cytoplasmic mass is of course the form of the cell 

 as a whole, as described above. 



Other Bodies in the Cell. — As indicated in Fig. 20, a cell may possess 

 other features. There is usually, perhaps always, a differentiated portion 

 at the surface, whose nature is different in different cases, and which is 

 variously' designated. Often it is a lifeless secretion of the protoplasm, in 

 which case it is called a cell wall. In plants and in some of the Protozoa, 

 this cell wall is composed of cellulose, a substance related to the starches, 



Fig. 20. — Diagram of a typical cell, with structures commonly present, at, attraction- 

 sphere; ch, chromatin net- work; c?, cell inclusions; cm, cell membrane; en, centrosome; cy, 

 cytoplasm; I, linin thread; n, nucleus; nl, nucleolus; nvi, nuclear membrane; ns, nuclear 

 sap; p, plastids; pdiv, plastid dividing; v, vacuole. 



but other substances as lignin or silica are often associated with it. In 

 many Protozoa the wall is composed of nitrogen-containing substances 

 resembling chitin, keratin, and gelatin. 



Instead of a wall the surface of the cell may be covered with a thin, 

 filmy sheet, called the pellicle or cuticle, the nature of which is uncertain. 

 Probably the pellicle is to be regarded as a hfeless secretion, as is the 

 cell wall. Such pelhcles are found in the Infusoria (ciUated Protozoa) 

 as described above (Fig. 18). 



In cases where there appears to be no special covering, the surface 

 layer of protoplasm is differentiated, at least with regard to its physical 

 and probably also its chemical properties. Depending upon their effect 



