14 



INTRODUCTION 



The mesothelial coelDink^pouclies, as left above, ar e near th e 

 dorsal side_of_the^embr^. 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 (hjrpomere, km), and a middle-plate 

 zone (mesomere, mm) between them. All three of these occur in 



Fig. 8. — Stereogram of a later stage than fig. 6, showing the segmentation of the 

 mesothelium. 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; c, ccxlom; 

 em, epimere; fb, fore-brain; hb, hind-brain; hm, hypomere; m, myotome; mb, mid-brain; 

 mm, mesomere; tnc, metacoele; myc, myocoele; «, nervous system; nc, notochord; s, 

 stomodeal region; sk, sclerotome. (In many vertebrates the sclerotome extends upward 

 to the lower part of m.) 



the trunk, but only the epimere is well developed in the anterior part 

 of the head. 



A series of vertical constrictions, beginning on the dorsal margin 

 of each coelomic pouch, cut down through epimere and mesomere 

 (fig. 8) so that the whole may be compared to a glove with many 

 fingers, each finger being hollow and all of the cavities connecting 

 with that of the hypomere (palm). This segmentation process 

 begins in front and gradually extends backward. Next, the dorsal 

 (epimeral) part of each of the fingers is cut from the rest, thus form- 

 ing a series of hollow cubes known as myotomes, each with a part of 

 the coelom, the myocoele, in its interior. ~^ VTewed from above, these 



