THE MULTICELLULAR ANIMAL 61 



lines the gastrula cavity (enteric cavity), as the endoderm. 

 The ectoderm comprises cells which are already somewhat differen- 

 tiated among themselves for special purposes, but which, as a 

 whole, form a primary tissue with general functions of its own, 

 chiefly sensory and locomotor. Similarly the endoderm consists 

 of cells which, as a group, form the nutritive cells of the embryonic 

 animal. (Fig. 31.) 



In the gastrula stage of most animals, a third layer of cells arises 

 from the endoderm and becomes disposed between the ectoderm 

 and endoderm. This new middle layer is the mesoderm. In this 

 wav the so-called three primary germ layers are established 

 which are characteristic of the developing animal, and from these 

 are derived the specialized tissues which compose the various 

 organs of the adult. For example, the ectoderm by cell division 

 and differentiation gives rise to the outer skin and central nervous 

 system; the mesoderm to muscular and supporting tissues and 

 the blood vascular system; while the endoderm forms the layer 

 of cells which lines the alimentary canal of the adult organism. 



B. Tissues 



This grouping of more or less similar cells into functional sys- 

 tems, or tissues, is at the basis of the architecture of multicellular 

 organisms, and thus we have now reached another level in the 

 analysis of their structure. Although the unit of organization is 

 the cell, these are associated in groups, or tissues, which represent 

 a morphological unit of a higher order — a division of labor among 

 the cells that makes possible the multicellular body with its attend- 

 ant complexity. A tissue may be defined as a group of essentially 

 similar cells specialized to perform a certain function. It is con- 

 venient to distinguish six main groups of animal tissues: epithelial, 

 supporting, muscular, nervous, circulating, and germinal. (Figs. 4, 

 32, 34.) 



The importance of epithelial tissues is evident from the fact 

 that the body is covered and lined with these cellular membranes 

 so they form the point of contact between the organism and its 

 environment. For example, food before it is really inside the body 

 must pass through an epithelium lining the digestive tract, and 

 before it can do that it must be digested by enzymes secreted by 

 epithelial cells. Furthermore, the waste products of metabolism 

 must be excreted and pass from the body through epithelial mem- 



