EMBRYOLOGY. 13 



teron in connexion with the exterior. A similar, but less well defined 

 proctodeum (fig. 7, p) arises at the hinder end of the digestive tract. 

 Thus the anterior and posterior ends of the alimentary canal are 

 ectodermal, the middle region entodermal, in origin. 



Metamerism. In the adult, various parts, essentially like each 

 other, are repeated one after another are metameric. The list 

 includes, among others, muscles, nerves, blood-vessels, vertebrae, ribs, 

 etc. There is much evidence to show that metamerism had its origin 

 in the mesothelial structures and has been secondarily impressed on 

 other systems. 



FIG. 8. Stereogram of a later stage than fig. 6, showing the segmentation of the meso- 

 thelium. The approach of the walls of the coelom (c), dorsal and ventral to the alimentary 

 canal, to form the mesenteries is shown, al, alimentary canal; em, epimere ;/&, forebrain; 

 hb, hind brain; hm, hypomere; m, myotome; mb, midbrain; mm, mesomere; me, metacoele; 

 myc, myocoele; n, nervous system; nc, notochord; 5, stomodeal region; so, sp, somatic and 

 splanchnic layers; st, sclerotome. 



The mesothelial ccelomic pouches, as left above, are near the dorsal 

 side of the embryo. With growth they gradually extend downward 

 on either side and tend to enclose the whole archenteron, and upward 

 on either side of the notochord and spinal cord (fig. 8). The fates of 

 the different parts of the mesothelial walls warrants the recognition of 

 three horizontal regions or zones in the walls of each coelom. These 

 are a dorsal muscle-plate zone (epimere, em), a lower or lateral-plate 

 zone (hypomere, km), and a middle-plate zone (mesomere, mm) 

 between them. All three of these occur in the trunk, but only the 

 epimere is well developed in the anterior part of the head. 



