EMBRYOLOGY. 13 
teron in connexion with the exterior. A similar, but less well defined 
proctodeum (fig. 7, ») 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, vertebrz, 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. 
Fic. 8.—Stereogram of a later stage than fig. 6, showing the segmentation of the meso- 
thelium. The approach of the walls of the ccelom (c), dorsal and ventral to the alimentary 
canal, to form the mesenteries is shown. al, alimentary canal; em, epimere; fb, forebrain; 
hb, hind brain; hm, hypomere; m, myotome; mb, midbrain; mm, mesomere; mc, metaccele; 
myc, myoccele; m, nervous system; 7c, notochord; s, stomodeal region; so, sp, somatic and 
splanchnic layers; st, sclerotome. 
The mesothelial coelomic 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, fm), 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. 
