REPRODUCTION 89 



constitutes the essential secreting and absorbing layer of the tube, the 

 digestive epithelium. 



It is a noteworthy fact that various organs which have nothing directly 

 to do with digestion have their origin in the enteric endoderm. The 

 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 

 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. 



The number of pairs of pharyngeal pouches varies from a maximum 

 of fourteen in some cyclostpmes to, in birds and mammals, four or five 

 which are rudimentary in the sense that they exist only temporarily in the 

 embryo and even then may not perforate to the exterior. The clefts of 

 the first pair do not ordinarily produce functional gills in fishes but 

 become the spiracles of sharks and some ganoids. In all amniotes the 

 pharyngeal pouches are merely temporary embryonic features except 

 as those of the first pair are, in a modified way, represented in the auditory 

 passages. 



Lungs develop by formation of a mid-ventral pouch in the floor of the 

 pharynx. As the pouch enlarges, it bifurcates forming right and left 

 lungs. The entire epithelial lining, being the essential respiratory mem- 

 brane, of the adult lung is endodermal and continuous, by way of the 

 lining of bronchi and trachea, with the lining of the digestive tube. In 

 birds the respiratory cavities are continued beyond the lungs into a system 

 of air sacs, branches of which extend into various parts of the body 

 including even the cavities of some of the bones. It seems an extra- 

 ordinary fact that a bird's humerus — less often the femur — may contain 

 an air tube whose wall is derived from and is still continuous with the 

 enteric endoderm. 



The air-bladders (swim-bladders) of fishes are endodermal sacs 

 which grow out from an anterior region of the embryonic enteron. 

 They are usually dorsal, rarely lateral, or ventral as in the ganoid 

 Polypterus. 



