134 NEURULATION AND EARLY ORGANOGENY 



Aortic arches 

 Visceral pouch I 

 Viscerol groove I 



Eustachian tube 

 (visceral pouch I) 



Visceral pouch II 



Visceral groove I 



Visceral pouch I 

 Visceral groove II 

 Visceral pouch IV 



Visceral groove IV 

 Visceral pouch V 

 Visceral groove V 



Lung bud 

 Oesophagus 



EARLY STAGE 



Branchial cleft I 

 Operculum 



Branchial cleft II 

 Branchial cleft III 



Branchial cleft IV 



LATER STAGE 



Development of the respiratory systems of the frog larvae. 



development of most, if not all, of the higher vertebrates, including 

 man. This region of approximation of the folds is sometimes unfor- 

 tunately called the "primitive streak" because of certain homologies 

 with similarly developing structures in the chick embryo. The blasto- 

 pore at the posterior end of the neurocoel is now completely closed, 

 but in the meantime there has developed a new invaginating pit just 

 ventral to the blastopore, known as the ectodermal proctodeum. 

 Occasionally the closing of the dorsal blastopore and the opening 

 of the ventral proctodeum are connected by the aforementioned 

 "primitive streak." The proctodeum is the primordium of the anus, 

 and establishes a new ectodermally lined opening into the hindgut. 

 The extent of the proctodeal ectoderm can be determined in sagittal 

 sections by determining the limit of the invaginated and pigmented 

 ectodermal cells. 



The body of the neurula stage is laterally compressed along the 

 dorsal surface but ventrally the belly region bulges with the large 

 yolk endoderm cells. The tail bud is formed by a backward growth of 



