DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



119 



meets and fuses with its fellow of the opposite side in the midventral . line 

 caudal to the oral fossa (Fig. 96). The maxillary process still bounds the 

 oral fossa laterally. Meanwhile a broad downward projection from the 

 front of the fore-brain region the naso-frontal process comes in contact 

 laterally with the maxillary process (Fig. 96). Along the line of contact a 

 furrow is left, which extends obliquely upward to the eye rudiment and is 

 known as the naso-optic furrow. 



The various structures that have been mentioned bound the oral fossa 

 which has now become a deep quadrilateral pit. Cranially (above) the fossa 



Mid-brai 



Cerebral hemisphere 



Lat. nasal process 

 Nasal pit 



Med. nasal process 

 Angle of mouth 



Eye 



Naso-optic furrow 



Maxillary process 

 Mandibular process 

 Branchial grooves 



<jl| Branchial arch II 



FIG. 97. Ventral view of head of 11.3 mm. human embryo. Rabl. 



is bounded by the broad, rounded, unpaired naso-frontal process; caudally 

 (below) it is bounded by the paired mandibular processes; laterally by the 

 paired maxillary processes. Between the maxillary and mandibular pro- 

 cesses on each side a notch marks the future angle of the mouth. In general 

 it may be stated that the naso-frontal process gives rise to the nose and 

 middle of the upper lip, the maxillary processes to the lateral parts of the 

 upper lip and the cheeks, the mandibular processes to the lower jaws, chin 

 and lower lip. 



The naso-frontal process extends farther downward toward the mandi- 

 bular processes, so that the oral fossa becomes more nearly enclosed and the 

 entrance to it reduced to a slit. At the same time two secondary processes 



