FROM TWELVE TO THIRTY-SIX SOMITES 173 



alimentary tract and the appendages proper to each in the follow- 

 ing order: (1) Stomodseum, (2) Pharynx, (3) CEsophagus, (4) 

 Stomach, (5) Hepato-pancreatic division of the fore-gut, (6) Mid- 

 gut, (7) Hind-gut. 



The stomodaeum owes its origin to an expansion of the em- 

 bryonic parts surrounding the oral plate, and it gives rise to a 

 large part of the buccal cavity, which is therefore lined by ecto- 

 derm. (See Chap. X.) It will be remembered that at the 12 s 

 stage the oral plate lies between the pericardium and the fore- 

 brain (Fig. 67), and that it consists of a fusion between the 

 ectoderm of the ventral surface of the head and the entoderm 

 composing the floor of the anterior end of the fore-gut. It lies 

 in a slight depression on the under surface of the head which 

 is the beginning of the oral cavity. This small beginning owes 

 its enlargement (1) to the cranial flexure, by which the ventral 

 surface of the head becomes bent at right angles to the oral 

 plate instead of forming a direct continuation of it, and (2) to 

 the formation and protrusion of the mandibular arches and 

 maxillary processes at the sides and behind. (See fuller account 

 in Chap. VII.) In this waj' it becomes a deep cavity closed 

 internally by the oral plate. The series of figures of sagittal 

 sections through the head illustrates very well the gradual deep- 

 ening of the stomodseum by these processes (Figs. 75, 85, 87, 88). 



The oral plate thins gradually from the 12 to the 30 s stage 

 when it breaks through (Figs 87 and 88), thus establishing an 

 opening into the alimentary tract. The remnants of the oral 

 membrane then gradually disappear and leave no trace. The 

 subsequent extension of the maxillary region to form the upper 

 jaw greatly enlarges the extent of the ectodermal portion of the 

 buccal cavity. It will have been noted (Figs. 85 and 87) that 

 the hypophysis opens in front of the oral plate on the ectodermal 

 side, and this constitutes a most important landmark for deter- 

 mining the limit of the ectodermal portion of the buccal cavity 

 in later stages. 



The Pharynx and Visceral Arches. The pharynx may be briefly 

 defined as the alimentary canal of the head. It is the most 

 variable part of the alimentary canal in the series of vertebrates. 

 ]\Iodified, as it is in all vertebrates, for purposes of respiration, 

 the transition from the aquatic to the terrestrial mode of respira- 

 tion brought about great changes in it. It is thus marked em- 



