122 



METAZOA IN GENERAL 



From the entoderm is derived the epitheUum Uning the digestive 

 cavity or caual, except at the open ends; also the epithehum hning all 

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

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



^ Nerve cord 

 Mesoderm^ — / .chorda 



Archenferon 



Fig. 48. — Stages in the development of amphioxus, one of the lower chordates. (Drawn 

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

 showing a cre\dce 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 i.s 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 ventral ectoderm 

 growing over the blastopore and upon the dorsal surface, 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 archen- 

 teron, and the mesoderm developing from dorsolateral outpocketings of the entoderm. / 

 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 

 lining 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 



