EMBRYOLOGY 



13 



The Notochord. — Immediately beneath the neural plate is an 

 axial strip of entoderm^ (fig. 5, ch), bounded on either side by the 

 outgrowing mesothelium. When the latter separates (p. 10) this 

 band is momentarily rejoined to the rest of the entoderm but is still 

 recognizable from its different cells. It soon rolls into a rod (a tube 

 in some amphibians and birds), is cut off from the rest (fig. 6, n) and 

 lies between the digestive tract and the nervous system where it 

 forms an axis around which the skull and vertebral column develop 

 later. 



The Digestive Tract. — After the separation of the notochord, the 

 entoderm forms a tube, closed in front and usually behind as well. 

 The anterior end of the tube 

 abuts against the ectoderm of 

 the ventral side of the embryo. 

 Later the ectoderm grows in at 

 the point of contact, carrying the 

 entoderm before it and forming 

 a pocket, the stomodeum, which 

 gives rise to the cavity of the 

 mouth (fig. 212, oc). (In some , , ,. , 



, J 1 • 1 • r Fig. 7. — Schematic section of the hinder 



the Stomodeal mgrowth is at first end of an amphibian embryo, showing the 



relations of the neurenteric canal, ac, ali- 

 mentary canal; ec, ectoderm (black); w, 

 ^ Eventually notochord; we, neurenteric canal; nt, neural 



.1 , ' 1 " J i J t tube; p, proctodeum; pa, post-anal gut; 



the ectoderm and entoderm fuse y^ yoikf > f > y 



at the bottom of the cup, and 



then the fused area breaks through, placing the archenteron in con- 

 nexion with the exterior. A similar, but less well-defined procto- 

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

 Thus the anterior, and posterior ends of the ahmentary 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, vertebras, 

 ribs, etc. /There is much evidence to show that metamerism had 

 its origin in the mesothelial structures and has been secondarily 

 impressed on other systemsV 



' The statement is made that in some groups the notochord arises from another 

 germ layer than the entoderm, but these statements apparently rest on erroneous 

 observations or interpretations. Different origins in different vertebrates would tend 

 10 show that what are called notochord are not homologous. 



solid, the pocket being formed 

 later by splitting.) 



