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METAZOA IN GENERAL 



From the entoderm is derived the epitheUum hning the digestive 

 cavity or canal, except at the open ends; also the epithelium lining all 

 hollow structures formed as outpocketings of this cavity or canal. This 

 latter category includes, in the air-breathing vertebrates, not only the 



D Nerve cord 



Mesoderm 



Entoderm 



F hferve cord 



, chord- 



Nerve cord 



Fig. 51. — Stages in the development of amphioxus, one of the lower chordates. {Drawn 

 from Ziegler models, based on the work of Hatschek.) A, the four-cell stage, polar view, 

 showing a crevice between the cells. B, cross section through the opposite cells of an 

 eight-cell stage, showing a median space. C, a median section of the 32-cell stage; the 

 median space is developing into a blastocoel and the embryo is becoming a blastula without 

 passing through the morula stage. D, cross section of the blastula. E, invagination. F, 

 the gastrula has become asymmetrical and has turned on its side; the dorsal surface is 

 flattened, the ventral convex. G, the diploblastic embryo, showing the ectoderm growing 

 over and covering in the dorsal ectodermal cells which will form the central nervous system. 

 H, cross section of a later stage, showing the chorda, or notochord, arising as a median 

 dorsal outpocketing of the wall of the archenteron, and the mesoderm developing from 

 dorsolateral outpocketings of the entoderm. 7, cross section of a later stage, showing the 

 central nervous system, the chorda, and the enteron in the median line, and on each side a 

 mesodermal pouch, containing a coelomic cavity. The walls of this pouch are mesothelium. 



linings of cavities of such organs as the liver and pancreas but also the 

 hning of the so-called respiratory tract, consisting of the lungs and 

 the passageways leading from the pharynx to them. 



From the mesoderm are developed all of the other tissues of the body, 

 including muscles, connective and supporting tissues, the blood vessels 



