Chap. 19 DEVELOPMENT 375 



a ventral outpocketing of it near its front end is the first appearance of the 

 liver. At the posterior end the endoderm grows outward and the ectoderm in- 

 ward till they meet and break; the latter forms the lining of the future cloaca 

 and its external opening. At the anterior end, a similar ingrowth of ectoderm 

 which will line the greater part of the cavity of the mouth meets the endoderm 

 in an oral plate which also breaks through. Thus, the saclike enteron becomes 

 a tube. 



Only the linings of the alimentary canal and its branches are endoderm. In 

 various regions of these, cells are gradually differentiated for their respective 

 functions, such as secretion and absorption. Except for the nerves, mesoderm 

 composes the whole outer wall of the digestive canal and its derivatives such 

 as pancreas and liver and their ducts. The endodermal cells lining the finer 

 branches of the liver ducts become the cells which secrete the bile. Like the 

 liver, the pancreas also arises as an outpocketing of the inner layers of cells in 

 the wall of the digestive canal. 



The fundamental processes of ingrowth, outgrowth, and differentiation of 

 cells are repeated over and over again in all embryos. 



Respiratory System. The respiratory organs of vertebrates are also derived 

 from the digestive canal. Whether their function demands exposure to water 

 or air, their surfaces are continually moist and are always close to the blood. 



The first signs of a respiratory system in the tadpole are the outpushings 

 from the endoderm of the foregut, the region of the future pharynx (Fig. 

 19.10). There are in all six of these gill pouches on each side. The first and 

 last never open but about the time of hatching, the others meet the superficial 

 ectoderm, break, and become the gill clefts that give free passage to the water 

 outside. The solid bars of tissue anterior and posterior to the gill clefts are the 

 gill arches that support the gills. In the frog, the tissue in front of the first pair 

 of pouches that remain closed will form the lower jaw. In all vertebrates, these 

 pouches become the middle ears and the eardrum develops where the endo- 

 derm of the pouch meets the skin ectoderm which will line the tube of the 

 external ear. The Eustachian tube is derived from the part of the pouch nearest 

 the foregut and thus the pharynx and middle ear communicate (Fig. 17.9). 



In a newly hatched tadpole, respiration is carried on by external gills that 

 develop as outgrowths of the skin ectoderm of the three arches (Fig. 19.10). 

 These external gills are later absorbed and replaced by internal gills which 

 also arise from the gill arches. At about this time, a fold of ectoderm, the 

 future operculum, arises in front of the gill clefts and grows backward, form- 

 ing a mantle around the internal gills and gill arches of both sides (Fig. 

 19.10). It has one external opening on the left side, the only exit for the water 

 that enters the mouth and flows over the gills as the tadpole breathes. Even 

 before hatching, the lungs appear as two small outpocketings from the floor 

 of the future esophagus and are inconspicuously present through the period 



