METAZOAN ORGANIZATION 55 



The germ cells are set aside early in the life of the individual for 

 reproductive purposes. They develop in the reproductive glands or 

 gonads of the two sexes. The protoplasm of these cells is known as 

 germ plasm. The female germ cells are eggs or ova, and those of the 

 male are spermatozoa. When the germ cells reach maturity, they be- 

 come separated from the body and may give rise to a new generation. 

 About forty years ago Weismann presented the idea of the continuity 

 of heredity from generation to generation by way of the germ plasm. 

 The germ plasm, according to this idea, gives rise not only to the 

 protoplasm of the germ cells of the new individual but to the somatic 

 cells as well. In Protozoa the entire material of the individual is 

 passed on to the two offspring and, for this reason, this protoplasm 

 is spoken of as being immortal. Potentially, germ plasm is likewise 

 immortal. 



The protoplasm of the somatic cells is known as somatoplasm. This 

 is rebuilt with each generation, and when the individual dies, all of 

 the somatoplasm perishes. In final analysis, the somatoplasm serves 

 as a means of conveyance for the germ plasm through the current 

 generation. 



Cellular Organization 



The simpler Metazoa are composed of only two kinds of somatic 

 cells. These cells are grouped according to kind in two layers. 

 With advanced differentiation, a rather wide variety of cells has 

 been produced. 



A tissue is an organization of similar cells into a group or layer 

 for the performance of a specific function. A certain amount of 

 intercellular substance is characteristic of most tissues and enhances 

 their usefulness. The entire living mass of the metazoan animal body 

 may be classified under five fundamental (four by some authors) 

 kinds of tissues, and when it is so distributed, there is nothing left. 

 These classes of tissues are: epithelial, protective or covering; siis- 

 tentative, connective or supporting ;., muscular, contractile ; nervous, 

 irritable or conductive; vascular, circulatory. 



Epithelial Tissue. — A sheet of cells that covers external or in- 

 ternal surfaces of the body is known as an epithelium. The epi- 

 dermis or outer layer of the skin and the layer of column-shaped 

 cells lining the inside of the intestine are good examples. Accord- 

 ing to function, this type of tissue can be classified as protective 

 epithelium, glandular epithelium, and sensory epithelium. The epi- 

 thelium which covers external surface of an organism usually de- 



