; 
q 
-_ 
THE MOUTH AND NOSE. 37 
It has already been pointed out (p. 32) that the stomatodieal depression lies 
between the anterior part of the head (7.e. the tissues forming the base of the 
primary fore-brain) and the pericardial region, and that it is separated posteriorly 
from the fore-gut by the bucco-pharyngeal membrane. At first it has no distinct 
lateral boundaries, but subsequently the mandibular arches, which are developed at 
the sides of the bucco-pharyngeal membrane, project forward beyond it and form 
lateral limits of the depression. If the stomatodeeal space is examined from the 
front at this period the following boundaries are recognisable. Above and in front 
Maxillary 
Mesencephalon process Mandibular areh Prosencephalon 
Otic 
vesicle Hyoid arch 
Thalamen- 
cephalon 
Prosen- 
cephalon 
Olfactory 
De Olfactory 
pit 
Mesial 
| II nasal 
process 
Globular process 
ee 
Olfactory pit 
Mesial nasal_ 
process ~_ Tateral Cerebral 
hemi- 
nasal ; 
sphere 
process 
Lateral 
nasal 
process 
, Olfactory 
Q@ Pg¥) pit 
e747“ Mesial 
>” — nasal 
process 
"Globular 
process 
Stomatodzeum 
Fia. 29. 
I. Side view of the head of human embryo about 27 days old, showing the olfactory pit and the visceral arches 
and clefts (from His). 
Il. Transverse section through the head of an embryo, showing the relation of the olfactory pits to the fore- 
brain and to the roof of the stomatodeal space. 
III. Head of human embryo about 29 days old, showing the division of the lower part of the mesial frontal 
process into the two globular processes, the intervention of the olfactory pits between the mesial 
and lateral nasal processes, and the approximation of the maxillary and lateral nasal processes, which, 
however, are separated by the oculo-nasal sulcus (from His). 
IV. Transverse section of head of embryo, showing the deepening of the olfactory pits and their relation to the 
hemisphere vesicles of the fore-brain. 
is the projecting anterior part of the head which is termed the fronto-nasal process, 
laterally are the mandibular arches, and below and posteriorly is the anterior part 
of the pericardial region. After a short time the lower ends of the mandibular 
arches meet in front of the pericardial region, and, fusing together, form the 
posterior or lower margin of the aperture; simultaneously the lateral boundaries of 
the space are still further completed by the forward growth of a nodular projection, 
the maxillary process, from the upper end of each mandibular arch. About the 
fifteenth day the bucco-pharyngeal membrane disappears, and the stomatodzeal space 
and pharynx are thenceforth continuous. No trace of the bucco-pharyngeal mem- 
brane is recognisable in the adult, but its position may be represented by an 
imaginary plane extending from the anterior part of the basi-sphenoid above to the 
base of the alveolar process of the lower jaw, on its lingual surface, below. 
