METAMERA. 269 



specialized head of the Vertebrate with skull and brain is 

 a non-essential, secondary formation ; and the same may be 

 said of the limbs or extremities. Important as these parts 

 of Man and the higher vertebrates are physiologically, they 

 are morphologically unimportant, for originally they were 

 absent, and they develop only at a later period. The older 

 Vertebrates of the Silurian Period had neither skull nor 

 brain, and were entirely without limbs. 



If we pay no attention to those parts which are second- 

 arily formed, and are therefore unimportant, and if we 

 provisionally examine only the essential, primary parts, we 

 shall greatly simplify our task. This task is essentially 

 to trace the described organism of the "primitive Verte- 

 brate " from the simple germ-form of the Gastrula. That 

 simplest Vertebrate body is, as is usually said, composed of 

 two symmetrical, double tubes ; of a lower tube, the body- 

 wall, which surrounds the intestinal tube, and of an upper 

 tube, spinal canal, which surrounds the spinal marrow. 

 Between the spinal tube and the intestinal tube, lies the 

 notochord, the most essential part of the inner axis of 

 the skeleton which characterizes the Vertebrate. This 

 characteristic arrangement of the most important organs 

 re-occurs in all Vertebrates from the Amphioxus to Man. 

 (Cf. Plate IV., with explanation.) We must, therefore, 

 now examine the way in which these organs develop from 

 the two primary germ-layers of the Gastrula, and from the 

 four secondary germ-layers which arise by fission of the two 

 primaries. 



In order to solve this difficult problem it seems desirable 

 to begin with a statement of the most important conclusions 

 of ontogenetic study. The distant goal will be more easily 



