3.8 The branchial arches in a 5-mm human embryo. At 8 weeks the oral cavity is separated from the nasal cavity and the nasolacrimal groove is reabsorbed the morphology of the face is developing.įig. This membrane then ruptures and forms the primitive choanae. A thin oronasal membrane is located between the pits and the oral cavity. The nasal pits grow, approaching the primitive oral cavity. The nasal pits invaginate by forward growth of the medial nasal and lateral nasal processes, and by posteroinferior growth of the pits themselves, the placode tissue comes to line the roof of each pit. The nasal placodes begin to invaginate by the 5th week, forming the nasal pits. This furrow is called the nasolacrimal groove. The lateral nasal process is separated from the maxillary process (the more rostral portion of the first branchial arch) by a furrow that reaches the medial aspect of the developing eye. The medial portion of these eminences is called the medial nasal process the lateral portion is called the lateral nasal process. Nasal pits are located on either side of the frontonasal prominence and are surrounded by horseshoe-shaped eminences. This prominence is the frontonasal prominence. From 5 to 7 weeks a median prominence overhangs the cephalic end of the oral cavity. Development of the head from the 5th to the 8th week. Table 3.1 Stages of the development of the three ear sectionsĬartilaginous portion of the external auditory canalĬochlear sensorial cell membranous labyrinth otic capsule ossificationįig. The pharyngeal arches appear between the 28th and the 32nd days ( ▶ Fig. 3.7) optic vesicle and nasal placode appear between the 31st and 35th days. Already between the 25th and the 28th days neuropores are closed and the neural tube is enclosed inside the cephalic end with the appearance of the otic placode ( ▶ Fig. 1– 3 The first stages of ear development are very precocious, during the first half of the embryonic period, and are next only to those involving the formation of the cardiovascular system and the primitive placenta (days 21–22). Prenatal development can be divided into a first period (preembryonic period) of cellular zygote division, which includes the 1st and 2nd weeks post conception a second period, which runs from the 3rd to the 8th week, the definitive embryonic period and a third period, from the 9th to the 38th week, the fetal period ( ▶ Fig.
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