REPRODUCTION 



65 



anterior region of the embryonic enteron — the part becoming the pharynx 

 of the adult — is concerned particularly with the organs of respiration. 

 Gills of fishes and amphibians develop in relation to paired apertures, 

 the pharyngeal or visceral clefts, which pierce the lateral walls of the 

 enteron and the ectoderm and open to the exterior. A pharyngeal cleft 

 is developed as follows. A deep lateral pouch or furrow of the endoderm 

 bulges outward and meets a similar but shallower pouch or furrow which 



EC NT NC EN 



Fig. 57. — Frog: median longitudinal sections of embryos; .4, just before conversion 

 of blastopore into neurenteric canal; B, just after formation of neurenteric canal and 

 perforation of proctodeum to form cloacal aperture. B, brain; BP, blastopore; C, 

 cloacal aperture; EC, ectoderm; EN, endoderm; H, hypophysis; HT, heart; MES, 

 mesoderm; NC, notochord; A'^jE, neurenteric canal; AT, neural tube; O, region where 

 mouth will perforate; P, proctodeum; PH, pharynx; R, rectal region of enteron; Y, 

 yolk cells of endoderm; i, fore-brain; 2, mid-brain; 3, hind-brain. A, X24; B, XiQ. 

 (Redrawn from Marshall, "Vertebrate Embryology.") 



the ectoderm pushes inward. The resulting two-layered membrane 

 is then obliterated by the same process which removes the oral membrane, 

 leaving a free passage between the pharynx cavity and the exterior. 

 Vascular complications of the endodermal lining of these clefts produce 

 internal gills — although it is possible that some so-called internal gills 

 are derived from ingrowing ectoderm. External gills are ectodermal 

 structures developed in close relation to the external apertures of pharyn- 

 geal clefts. In amniotes the pharyngeal pouches are merely temporary 



