REPRODUCTION AND DEVELOPMENT IN CHORDATES 



pharyngeal region of the aUmcntary canal paired pouches extend to the sur- 

 face ectoderm in which sHts appear (Fig. 5.20). These gill slits or clefts 

 make possible a flow of water through the pharynx in adult fishes and certain 

 amphibians, including the tadpole of the frog. Gills are developed later in 

 this region. Such pharyngeal pouches and at least vestigial gill slits are 

 characteristic of all chordate embryos (Figs. 5.21 and 5.22). In terrestrial 

 vertebrates the lungs and arr tubes are also formed by an outgrowth from 

 the endoderm lining the pharyngeal region of the primitive gut. 



The mesoderm undergoes many changes in the period after its localization. 

 Most conspicuous is the proliferation that occurs along the sides of the neural 

 tube, forming what is called the dorsal mesoderm, or epimere. Soon these 

 cords of mesoderm become segmented and give rise to the mesodermal somites 

 (Figs. 5.17 and 5.21). Later, after continued cell division, the cells of the 

 somites are shifted in position. Some migrate around the notochord and 

 neural tube and later form the vertebral column. Others make up the muscle 

 plates, from which the striated muscles of the trunk arise, and still others 

 form the dermis of the skin. Lateral to the dorsal mesoderm on each side 

 there is a region called the intermediate mesoderm, or mesomere, from which 



Ectoderm Endoderm 



Neural crest 

 Neural tube 

 Notochord 

 Archenteron 



Coelom 



Brain 



Mesoderm 

 Dorsal surface 



Spinal cord 



Anus 



Anterior end 



Liver 



Pericardial cavity 



Fig. 5.19. Young tadpoles of the frog, to show localization of organ-system primordia. A, 

 tadpole of about hatching age, in longitudinal section. B, later tadpole, in cross section; 

 and C, later tadpole, in longitudinal section. All sections diagrammatic. (.4, redrawn from 

 T. H. Morgan, The Development of the Frog's Egg, copyright 1912 by The Macmillan Co., printed 

 by permission; C, redrawn from J. W. Jenkinson, Vertebrate Embryology, copyright 1913 by 

 Oxford University Press, printed by permission.) 



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