E. METAZOA. 



Metazoa are many-celled animals in which there is a differentia- 

 tion into at least two body layers, the Ectoderm and the Endodcnn; 

 the former is the organ of relation, the latter the organ of nutrition ; 

 in addition all have heteromorphic sex cells, ova and spermatozoa. 

 In all metazoa the fertilized ovum undergoes repeated divisions 

 (Cleavage) which lead up to the formation of a hollow sphere of 

 cells ( Blastula ) and from the latter arises a two layered condition 

 (Gastrula) the outer layer being the Ectoderm, the inner the Endo- 

 dcnn; between these two a third layer, the Mesoderm, is usually 

 formed. 



The cells of the different germinal layers differ from one 

 another, and in the course of further development differentiations 

 appear among the cells of the same layer. In this way arise Tis- 

 sues, differentiated groups of like cells and their products. From 

 the two primitive tissues, epithelium and mesenchyme, present in 

 the blastula and gastrula all other tissues are derived, as shown 

 herewith : 



i. EPITHELIUM 2. MESENCHYME 



a. Epithelial tissue a. Connective tissue 



b. Muscular b. Skeletal 



c. Nervous c. Vascular 



d. Germinal d. Storage ( reserve) ' 

 These different tissues will be studied in the laboratory in con- 

 nection with each animal considered. 



The various functions of animal life, which in the Protozoa are 

 all performed by a single cell, are performed in the Metazoa not 

 only by many cells and tissues but also by groups of different 

 tissues united to form Organs, each with a specific function, and 

 by groups of organs united to form Systems, each having some 

 one general function, as shown in the following table : 



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