CHAPTER LXII 



STRUCTURE OF ORGANISMS 



In the simplest types of one-celled organisms there is little distinction 

 between the different parts of the cytoplasm, all being freely interchange- 

 able and all being able to carry on the same activities. In the higher 

 Protozoa, owing to intracellular differentiation, certain portions of the 

 cytoplasm become set aside for the performance of particular functions 

 (Fig. 31). Since they are thus analogous to the organs of higher animals, 

 they have been designated by such names as cell organs and organelles. 



467. Grades of Organization. — Differentiation has already been 

 considered in Chaps. XIX, XX, and LXI, in which attention was called 

 to the fact that tissues are formed as a result of intercellular differentia- 

 tion. These become associated together to form organs, and organs 

 become related to each other to form systems. Four types of tissues 

 are generally recognized, and eight or nine systems of organs. It 

 was also seen in Chap. XXX that this organization was first fully 

 shown in the flatworms; in higher animals it has been found to reach 

 a high degree of complexity. Generally speaking, an organism is said 

 to be low in organization when its structure is comparatively simple and 

 high in proportion to its complexity. Accordingly the various phyla 

 show various grades of organization. 



468. Germ Layers and Tissues. — In the development of a metazoan, 

 as traced in Chap. XXV, it has been seen that differentiation first 

 results in the formation of three ge7'm layers. It has also been noted 

 that as the organism develops, these germ layers each give rise to certain 

 definitive tissues, which are mature tissues as contrasted with those found 

 in embryos or young animals. The classification of tissues depends, 

 however, not upon the germ layers from which they are derived but upon 

 the character which they possess. Thus epithelia may come from any 

 one of the three germ layers (Sec. 146) ; most muscle cells are mesodermal 

 in origin but in certain cases they are derived from ectoderm or entoderm ; 

 and the supporting framework of nerve tissue, called in general neuroglia 

 and ectodermal in origin, has the character and function of a connective 

 tissue, a type of tissue which is usually mesodermal, 



469. Relationship of Cells in Metazoans. — While the cells in a meta- 

 zoan may be considered structural units, they should not be thought of as 

 independent of one another. As a matter of fact many of the processes 

 which result in the formation of cells do not go on to a complete separa- 



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