THE MULTICELLULAR ORGANISM 57 



free. Thus we have a foreshadowing of that differentiation 

 and physiological division of labor between cells which is 

 the most characteristic feature of the Metaphyta and 

 Metazoa. (Figs. 18, 115.) 



However, in the developing multicellular organism cleav- 

 age results, sooner or later, in a body composed of cells 

 which possess differentiations of one kind or another that 

 adapt them for the special part they are destined to play in 

 the economy of the individual. Thus cell division, involving 

 differentiation, is the keynote of development in the higher 

 plants and animals. 



Among animals, for example, the cells which arise from 

 the cleaving egg frequently become arranged so that they 

 form the surface of a hollow sphere of cells known as a BLAS- 

 TULA. All the cells at first appear essentially similar, but 

 soon those at one side of the blastula become invagi- 

 nated until the central cavity, termed the BLASTOCOEL, is 

 largely obliterated. Accordingly there results the GASTRULA 

 stage, which may be roughly compared to a sack, with an 

 opening to the exterior termed the BLASTOPOBE, composed of 

 two layers of cells. The outer layer is known as the ECTO- 

 DERM, and the inner, which lines the gastrula cavity (ENTERIC 

 CAVITY), as the ENDODERM. The ectoderm comprises cells 

 which are already somewhat differentiated 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. 19.) 



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

 arises typically from the endoderm and becomes disposed 

 between the ectoderm and endoderm. This new middle 

 layer is the MESODERM. In this way the so-called three 



