FROM TWELVE TO THIRTY-SIX SOMITES 173 
alimentary tract and the appendages proper to each in the follow- 
ing order: (1) Stomodzum, (2) Pharynx, (3) Ctsophagus, (4) 
Stomach, (5) Hepato-pancreatic division of the fore-gut, (6) Mid- 
gut, (7) Hind-gut. 
The stomodeum 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 12s 
stage the oral plate lies between the pericardium and the head- 
fold (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 way 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 stomodeum by these processes (Figs. 75, 85, 87, 88). 
The oral plate thins gradually from the 12 to the 30s 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 
buceal 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. 
Modified, 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- 
