268 THE EVOLUTION OF MAK 



But, in order to perfect our picture, we must turn for a 

 moment to pay special attention to their articulation or 

 metameric structure, which is best seen in the longitudinal 

 section (Fig. 52, 53). The body of Man, as of all developed 

 Vertebrates, appears to be composed of a string or chain of 

 like members lying one behind the other along the longi- 

 tudinal axis of the body. In Man the number of these 

 like segments or metamera is about forty ; in many Ver- 

 tebrates, for example, in Snakes and Eels, it is several 

 hundred. As this inner articulation corresponds essentially 

 with the vertebral column and the muscles surrounding it, 

 these members, segments or metamera, are called primitive 

 vertebrae. Now, this structure of these primitive ver- 

 tebrae, or internal metamera, is correctly regarded as a 

 prominent characteristic of Vertebrates, and the various 

 forms into which it is differentiated bear greatly on the 

 different groups of Vertebrates. But in our present task, 

 that of tracing the development of the simple body of the 

 primitive Vertebrate from the Gastrula, the segments or 

 metamera are of subordinate significance, and we need not 

 deal with them till later. 



Putting these metamera temporarily aside, I think that, 

 in the above brief description of the essential parts, I have 

 said everything necessary as to the fundamental structure 

 of Vertebrates. The chief organs which have been men- 

 tioned are the original and most important parts, nearly all 

 of which are to be found, in a similar form, in the adult 

 Amphioxus, and which re-occur in the original rudimentary 

 germ of all members of this tribe. Many very important 

 parts, which appear to be entirely essential, will, it is true, 

 be missed in this review. As I have already remarked, the 



