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B1,B2:
Pharynx. Crest in ventral floor of pharynx formed
by fusion of 3rd pharyngeal arches = hypopharyngeal
eminence (precursor of root of tongue). Rathke's
pouch = rudimentary adenohypophysis.
B3:
Rudimentary thyroid ventral to aortic sac (also
seen in B2, ventral to the hypopharyngeal
eminence).
B4:
Caudal pharynx compressed dorsoventrally.
B7:
Glottis drawn off from pharyngeal foregut. Nasal
placodes. Pulmonary arteries.
C1,C2:
Commencement of trachea and oesophagus with dense
mesenchyme. R. nasal pit.
C3,C4:
Common cardinal vein in the posterior wall of the
intraembryonic coelom - the pleuropericardial folds
which contribute later to the formation of the
pleura and pericardium. L. nasal pit. In C4,
junction of R common cardinal vein with dorsal wall
of sinus venosus.
C5:
Smaller oesophagus, expanding trachea. Note ventral
anchoring of attachment site is at the most cranial
extension of the septum transversum. Note also that
this attachment
now divides the intraembryonic coelom around the
trachea into two canals, the L and R pleuro
(pericardio-peritoneal) canals. (Canals are lined
by coelomic mesothelium and are continuous with
whole I-E coelom - they will be referred to
hereafter simply as coelomic canals). Note the
pleuroperitoneal fold on the medial side of the R
common cardinal vein - this fold will form part of
the diaphragm.
C5,C6:
Lateral extension of pulmonary mesenchyme is
moulded to shape of coelomic canals. Oesophagus
lumen obliterated (common site of oesophageal
atresia and/or tracheo-oesophageal fistula).
Prominent R pleuroperitoneal fold.
C7:
Note dorsal extent of coelomic canals. Oesophagus
lumen reappears caudal to bifurcation. Distinct R
(smaller on L) pleuroperitoneal fold below the
common cardinal vein.
Dl:
Oesophagus/stomach junction. Coelomic canals.
D2:
Ovoid stomach with developing space of the lesser
sac on R. Dorsal and ventral attachments of the
mesenchyme are now known as dorsal and ventral
mesogastria. Coelomic canals.
D3:
Rotation of stomach (seen from above) to R side.
Note change in outline of coelomic canals due to
presence of liver. Lesser sac. Note thick
mesothelium lining the coelom along Ledge of
stomach, the primordium of the spleen and greater
omentum along greater curvature. Liver embedded in
septum transversum (ventral border of septum
transversum contributes to diaphragm).
G7:
Caudal pharynx (extending laterally, ventral to
dorsal aorta - cf B4). Stomach, mesentery.
G6:
Narrow oesophagus. Tracheal bifurcation dorsal to
sinus venosus.
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