GERM LAYERS 81 



entirely similar cells having a similar function, but as develop- 

 ment progresses the cells become differentiated in groups for 

 the performance of different functions, some cells of the ecto- 

 derm forming the outer covering or skin, the nervous system, 

 etc., while cells of the endoderm become differentiated for dif- 

 ferent processes of digestion. Tissues are aggregates of similar 

 cells having the same function. In the gastrula there are two 

 tissues, endoderm and ectoderm, but in later development 

 many different tissues are formed from these two, e.g., epithelial, 

 nerve, muscle, connective tissue and the like. In the majority 

 of higher animals a third germ layer, termed the mesoderm, is 

 formed between the ectoderm and the endoderm. This third 

 layer gives rise to muscle tissues, endothelial, supporting or 

 connective, and germinal tissues (see Chapter VI). Organs are 

 aggregates of tissues for the performance of one function; diges- 

 tive organs, the stomach, liver, etc., consist of secreting, muscu- 

 lar, nerve, vascular, and connective tissues. Organs finally are 

 grouped in systems for the performance of the fundamental 

 vital processes of metabolism. The digestive system includes all 

 of the organs necessary for the digestion of food; the muscular 

 and supporting systems, the organs of locomotion; the excretory 

 system, the organs for disposing of waste matters; the respira- 

 tory system, the organs for obtaining oxygen and removing CO2j 

 the nervous system, the organs for receiving and transmitting 

 stimuli, and the reproductive system, the organs for maintaining 

 the race. At the bottom of all of the complicated structures is 

 the single cell, the minute, active, and mysterious unit of living 

 matter. Cells form the tissues ; tissues form organs ; organs form 

 systems; and the systems working harmoniously together form 

 the normal living organisms. 



All types of metazoa start with an analogous egg-to-gas- 

 trula stage in development, and differentiation begins from this 

 point, although in many cases differentiation may begin even 

 before this. Naturalists divide the animal kingdom into great 

 groups, termed phyla, according to the degree and nature of the 

 differentiation which follows from this gastrula stage. The 

 simplest of the metazoa are those which depart least from the 



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