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A D V E R T I S E M E N T
A D V E R T I S E M E N T
(4mm, 27d, stage 12/13) In the fourth week of gestation primordia of the face appear at the cephalad end of the embryo. The bulbous frontal process is capped by two nasal discs. The optic discs appear posterolateral to the frontal process. Three paired branchial arches have formed. The first arches splits into maxillary and mandibular processes. The hyoid arches are the second pair. between the frontal process and first arches, the buccopharyngeal membrane becomes fenestrated.
A D V E R T I S E M E N T
(7mm, 33d, stage 14/15) In the fifth week, nasal pits develop in the nasal discs and the rims of the discs differentiate into lateral and medial nasal processes. The lens vesicles invaginate and close within the optic discs. The mesenchyme of the mandibular arch fills in across the midline. The caudal end of the medial nasal processes begin to fuse with the maxillary processes.
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A D V E R T I S E M E N T
(12mm, 37d, stage 16) At the beginning of the sixth week, the nasal have shifted to a more ventral, central position. Growing and shifting sub-ectodermal mesenchyme smoothes out the furrows between arches and processes and the second arch becomes more massive. Six auricular hillocks which will become the pinna of the ears form on the mandibular and hyoid arches.
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This page was last modified on 5/17/1999.
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