CHAPTER TWELVE 



TJae Endodermal Derivatives 



The Mouth (Stomodeum, Jaws, Lips, The Thyroid Gland 



Etc.) The Tongue 



The Foregut The Lungs 



The Pharynx The Liver 



The Thymus Gland The Pancreas 



The Carotid Glands The Oesophagus and Stomach 



The Parathyroid Glands The Midgut 



The Ultimobranchial Bodies The Hindgut 



The endoderm gives rise to all structures associated with the origi- 

 nal archenteron, from the mouth (ectodermal stomodeum) to the 

 anus (ectodermal proctodeum), and all of its derivatives. It must be 

 emphasized, however, that the endoderm contributes only the lining 

 epithelium of these structures and that (since each of them is invested 

 with blood, connective and nervous tissue, and often with muscle) 

 ectoderm and mesoderm may also be involved. It is nevertheless con- 

 venient to describe in sequence from the anterior to the posterior (fore- 

 gut, midgut, and hindgut) those structures whose linings are endo- 

 dermal in origin. 



The Mouth (Stomodeum, Jaws, Lips, Etc.) 



The lips and anterior lining of the mouth are ectodermal and the 

 margin between the ectoderm and endoderm can be determined in the 

 larva by the extent of the invaginated and pigmented stomodeal 

 ectoderm. This ectoderm plus the oral endoderm form the oral plate, 

 or oral membrane, which generally ruptures through shortly after the 

 time of hatching (6 mm. stage) to form the mouth opening. The 

 lateral margins of the mouth (stomodeum) are the original mandib- 

 ular ridges between which are the dorsal and the ventral (larval) 

 lips. These are transitory but important feeding organs of the tadpole. 

 The dorsal lip develops three medially placed rows of superficial teeth 

 which are periodically sloughed off and replaced. The ventral lip of 

 the larva develops four rows of somewhat more complete teeth, but 

 all teeth are covered with stomodeal ectoderm. A cornified ectodermal 



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