^^ THE NEW EVOLUTION "^^"^ 



117, 119, p. 185), which is radially symmetrical about 

 its only axis. If the gastrula should continue its 

 development to the adult stage, following to its 

 logical conclusion the preceding line of geometrical 

 development, the result would be an animal radially 

 symmetrical in form with the body composed of two 

 layers of cells. Such an animal type we actually 

 find in the sea-anemones, hydras and allied creatures 

 — the so-called coelenterates (figs. 3, 4, p. 5; figs. 78, 

 79, p. 143). 



But during the development of the gastrula some 

 irregularity might appear which would disturb the 

 fundamental radial symmetry either partially or com- 

 pletely, leading to the development of animal types 

 more or less elongated in form and with, instead of 

 radial, bilateral symmetry — that is, with the two 

 halves the same on either side of the central plane 

 (see figs. B, C, D, pp. 140, 146, and 2.50). 



Excepting for the protozoans, the sponges, the 

 coelenterates, and the ctenophores, all animals are 

 either completely bilaterally symmetrical, or the body 

 is mainly bilaterally symmetrical with more or less 

 marked traces of radial symmetry. Furthermore, 

 during their development they all pass through a 

 stage which is either a more or less typical gastrula 

 (figs. 116, 117, p. 185), or may be interpreted as repre- 

 senting and derived from the gastrula. No matter 

 how different they may be, the gastrula is common to 

 them all. They all develop in comparable fashion 

 as far as the gastrula, but from that point they all 

 diverge, each in a different direction. 



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