GERM LAYERS 79 



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 different 

 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, 

 muscular, nerve, vascular, and connective tissues. Organs 

 finally are grouped in systems for the performance of the 

 fundamental vital processes of metabolism. The digestive 

 system including all of the organs necessary for the digestion 

 of food; the muscular and supporting systems for locomotion; 

 the excretory system of organs for disposing of waste matters, 

 the respiratory system for obtaining oxygen and removing CO2, 

 the nervous system for receiving and transmitting stimuli, and 

 the reproductive system 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. 



Every^tv_T3e of metazoa starts with an analogous egg to gas- 

 trula stage in development and differentiation begins from this 

 point, although in many cases differentiation begins even before 

 this. Naturalists divide the animal kingdom into great groups 

 termed phyla according to the degree and nature of the differen- 

 tiation which follows from this gastrula stage. The simplest 

 of the metazoa are those which depart least from the gastrula 



