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C1-C7: Beginning of trachea, oesophagus. Nasal placodes (nasal pit forming on one side). Forebrain. note proximity of nasal pit to prosencephalon (future site of olfactory nerve). In C2-C4, identify the medial and lateral nasal swellings.
DEVELOPMENT OF RESPIRATORY SYSTEM & GIT1
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.
Lateral extension of pulmonary mesenchyme is moulded to shape of coelomic canals. R common cardinal draining directly into sinus venosus; L common cardinal vein with prominent L pleuropericardial fold. Oesophagus lumen obliterated (common site of oesophageal atresia and/or tracheo-oesophageal fistula). Prominent R pleuroperitoneal fold.
DEVELOPMENT OF THE NERVOUS SYSTEM
C1-5: Forebrain. Also identify the sulcus limitans separating the alar and basal plates.
