274 



ELEMENTS OF BIOLOGY 



endodermal tube which breaks through to form the mouth. In some 

 cases the anus is developed from the original blastopore but in the 

 higher animals where a definite open blastopore does not form 

 during gastrulation the anus develops as a break in the contact area 

 between the posterior end of the endodermal tube and adjacent 



fsr POUCH 



2nd pouch 



3 no POUCH 



4TH POUCH 



MOUTH 



THYROID 



TRACHEA 



LUNG BUD 



OESOPHAGUS 



Fig. 1 86. — Visceral pouches in the embryo of a vertebrate. 



ectoderm (Fig. 185). Thus is established the characteristic tube- 

 within-a-tube structure, the outer wall being developed from the 

 ectoderm and the outer or somatic layer of mesoderm and the inner 

 tube being formed by the endoderm and the layer of splanchnic 

 mesoderm which immediately surrounds it, anterior and posterior 

 openings forming as apertures in the area of contact between ecto- 

 derm and endoderm. 



