GASTRULATION AND CONCRESCENCE. 



37 



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FIG. 30. The embryo now presents typical vertebrate conditions. The old mouth is practically closed. The 

 posterior thoracic and vagus appendages appear as the external gills, and the pleural fold as the postcephalic 

 lateral fold that gives rise to the pectoral and pelvic appendages. 



FIGS. 31 and 32. Side views of Figs. 26 and 27, showing the extension of the germ wall over the yolk, and the 

 elevation of the procephalic lobes above the general level of the yolk. 



FIG. 33. Side view of Fig. 29. It shows the projection of the procephalic lobes beyond the anterior surface 

 of the yolk, thus allowing the oral arches, or the basal arches of the anterior thoracic appendages, to approach 

 the haemal surface of the forehead. The lateral eye has been carried into the brain vesicles, appearing through the 

 skin as the kidney-shaped retina. 



FIG. 34. Side view of Fig. 30. At least four pairs of oral arches have united on the haemal suface, giving rise 

 to the premaxillary, maxillary, mandibular. and hyoid arches. Vestiges of the free appendages persist as the 

 oral arch papillae, tentacles, or balancers. The large thoracic segmental sense organ opposite the fourth thoracic 

 appendage and behind the hyoid arch has now become the auditory placode. 



