62 



HUMAN ANATOMY. 



Fronto-nasal 

 process 



Olfactory 

 pit 



Primitive 

 oral cavity 



Mesial nasal process 

 Maxillary process 

 Mandibular process 



Head of human embryo of about twenty-seven days, 

 showing boundaries ot primitive oral cavity. X 7. 

 (After His.) 



dorsal expansion gives rise to the middle ear, while the occluding- plate separating 

 the outer and inner furrows supplies the tissue from which the tympanic membrane is 

 formed. The second furroAV in great part disappears, but its lower portion con- 

 tributes the epithelium of the faucial tonsil and the supratonsillar fossa. The fossa of 

 Rosenmiiller is a secondary depression and probably does not represent the original 

 furrow. The third and fourth pouches give rise to ventral entoblastic outgrowths 

 from which the epithelial portions of the thymus and of the thyroid body are developed 

 respectively. The last-named organ has an additional unpaired origin from the ento- 

 blast forming the ventral wall of the pharynx in the vicinity of the second visceral arch. 



^ The Development of the Face 



FiG^74- and the Oral Cavity. — The earliest 



suggestion of the primitive oral cavity is 

 the depfession, or stomodcEiim, which ap- 

 Laterai nasal process pears about the thirteenth day on the 

 ventral surface of the cephalic end of the 

 embryo immediately beneath the ex- 

 panded anterior cerebral vesicle. The 

 oral pit at first is separated from the ad- 

 jacent expanded upper end of the head- 

 gut by the delicate septum, the pharyn- 

 geal membrane, composed of the opposed 

 ectoblast and the entoblast, which in this 

 location are in contact without the inter- 

 vention of mesoblastic tissue. With the rupture of the pharyngeal membrane, the 

 deepened oral pit opens into the cephalic extremity of the head-gut, now known as 

 the primitive pharynx. 



The formation of the face is closely associated with the growth and fusion of 

 the upper visceral arches in conjunction with the surrounding parts of the ventral 

 surface of the head. The first visceral arch, as already described, presents two 

 divisions, the maxillary and the mayidibzilar process. The latter grows ventrally and 

 joins in the mid-line its fellow of the opposite side, to form, with the aid of the 

 second visceral arches, the tissues from which the lower boundary and the fioor of 

 the mouth are derived. The upper and 



lateral boundaries of the primitive oral Fig 



cavity and the differentiation of the nasal 

 region proceed from the modification and 

 fusion of three masses, the two lateral 

 paired maxillary processes of the first 

 visceral arches and the mesial unpaired 

 fronto-nasal process, which descends as a 

 conspicuous projection from the ventral 

 surface of the anterior part of the head. 

 The maxillary processes grow towards 

 the mid-line and, in conjunction with the 

 descending fronto-nasal projection, form 

 the lateral and superior boundary of the 

 primitive oral cavity (Fig. 74). Very 

 soon the development of the future 

 nares is suggested by the appearance of 



slight depressions, the olfactory pits, one on each side of the fronto-nasal process ; 

 these areas constitute part of the wall of the forebrain, a relation which foreshadows 

 the future close association between the olfactory mucous membrane and the cortex 

 of the olfactory lobe. 



During the fifth week the thickened margins of the fronto-nasal process undergo 

 differentiation into the mesial nasal processes, while coincidently the lateral portions 

 of the fronto-nasal projection grow downward as the lateral nasal processes, these 

 newly developed projections constituting the inner and outer boundaries of the 

 rapidly deepening nasal pits. The line of contact between the lateral nasal process 

 and the maxillary process is marked by a superficial furrow, the naso-optic groove. 



Lateral nasal process 

 Maxillary process 



First external vis 

 ceral furrow 



Second visceral arch 



Third Visceral arch 



Head of human embrvo ot about thirty-four days. 

 {After His.) 



Mesial nasal 

 process 



Mandibular 

 process 



XS- 



