. Cunningham's Text-book of anatomy. Anatomy. 50 HUMAN EMBRYOLOGY. processes being shut off from the margin of the ledge by the maxillary processes (Fig. 65). After the ledge is completed the dorsal ends of the olfactory pits are separated from the stomatodaeum by a thin membrane, but this soon disappears, and the pits open again into the stomatodseal space, through apertures which are called the primitive choanse. After the formation of the primitive choanas a ledge grows from the medial surface of each maxillary process towards the median plane, caudal to the choanal. These ledges, the palat
. Cunningham's Text-book of anatomy. Anatomy. 50 HUMAN EMBRYOLOGY. processes being shut off from the margin of the ledge by the maxillary processes (Fig. 65). After the ledge is completed the dorsal ends of the olfactory pits are separated from the stomatodaeum by a thin membrane, but this soon disappears, and the pits open again into the stomatodseal space, through apertures which are called the primitive choanse. After the formation of the primitive choanas a ledge grows from the medial surface of each maxillary process towards the median plane, caudal to the choanal. These ledges, the palatine processes, meet and fuse during the third month of foetal life, the fusion commencing ventrally and being completed dorsally in the region of the uvula. As the ledges meet and fuse, the stomatodneum is separated into a cranial and a caudal portion. The cranial part is the nasal cavity ; it is soon divided into two lateral halves by a septum which passes caudally from the base of the cranium. The caudal portion of the stoniatodamrn blends with the ventral part of the primitive pharynx to form the vestibule of the mouth and its deriva- tives, and the gums and teeth. The details of the process by which the primitive lips are separated into the permanent lips, and the gums are defined, are described in the section dealing with Hypophyseal depression the digestive System. Head of a Human The Derivative of the Proctodseum. The lips —The proctodaeurn is a surface depression which owes its origin to the elevation of the surface round the margin of the anal maxillary processes. membrane. It forms the lower portion of the pars analis recti of the adult. Urino-genital System.—The formation of the internal parts of the urino-genital system from the intermediate cell tract, the urino-genital chamber, and the differentiation of the external genitals in the region of the cloacal membrane are described in the account of the urino-genital system. The development of the auditory organ is
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectanatomy, bookyear1914
