50 



HUMAN EMBRYOLOGY. 



Anterior nasal orifice 



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 stomatodseum by a thin membrane, but this soon disappears, and the pits 

 open again into the stoniatodaeal space, through apertures which are called the 

 primitive choanse. 



After the formation of the primitive choanae a ledge grows from the medial 

 surface of each maxillary process towards the median plane, caudal to the choanae. 

 These ledges, the palatine processes, meet and fuse during the third month of fcetal 

 life, the fusion commencing ventrally and being completed dorsally in the region 

 of the uvula. As the ledges meet and fuse, the stomatodseum 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 

 stomatodseum blends with the ventral 

 part of the primitive pharynx and it 

 forms the vestibule of the mouth and its 

 derivatives, 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. 

 FIG. 66. PORTION OF THE HEAD OF A HUMAN The Derivative of the Proctodaeum. 



EMBRYO ABOUT 2 MONTHS OLD (His). The lips The proctodseum is a surface depression 

 :rXw7rm ^Mbie^thUatte 6 , ' T t which owes its origin to the elevation of 



palatine processes are growing inwards from the the Surface round the margin of the anal 



maxillary processes. membrane. It forms the lowest 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 so intimately associated with the 

 development of the pharyngeal portion of the primitive gut that a short considera- 

 tion of the chief phenomena may with advantage be introduced here ; but for the 

 details of the development of the internal; middle, and external portions of the ear 

 the student must refer to the account of the development given in association with 

 the description of the auditory organ. 



Palatine 

 process 



THE INTERNAL EAR, THE TYMPANUM AND AUDITORY TUBE, 

 AND THE EXTERNAL EAR. 



In the human subject, as in other mammals, the auditory organ consists of the 

 internal ear or labyrinth, the middle ear or tympanum, with which is associated the 

 auditory tube (O.T. Eustachian) ; and the external ear, which consists of the external 

 acoustic meatus with the auricle at its lateral end. 



The internal ear itself consists of two parts the cochlea, which is the true organ 

 of hearing, and the vestibule and the three semicircular canals connected with it, 

 which are associated with the recognition of alterations in the position of the head, 

 and, therefore, with the recognition and maintenance of equilibrium. 



The whole of the internal ear is lined with ectodermal epithelium, the auditory 

 epithelium, which is derived from the surface of the head of the embryo. It is 

 recognisable in embryos of about 2 '6 mm. (Fig. 67) as a thickened and slightly 

 depressed plate of ectodermal cells which lies on the surface of the head, in the 

 region of the hind-brain, dorsal to the second branchial cleft. As development 



