THE EMBRYO. 



1169 



it firmness and stability, and in each there is also found one of the primitive aortic 

 arches. Continuous with the dorsal end of the first arch and growing forward 

 from it is a triangular process, the maxillary process (Figs. 714, 716, 738). Ven- 

 trally it is separated from the mandibular arch by a V-shaped notch ; the first 

 branchial arch may therefore be said to divide into two, viz., the mandibular arch 



Lens. 



Olfactory 

 pit. \ 



Maxillary process. 



Mandibular arch. 



Hyo-mandibular cleft. 



Auditory vesicle. 

 Hyoid arch. 



Thyro-hyoid arch. 



Sinus 



-' prsecervicalis. 



FIG. 713. Profile view of the head of a human embryo, estimated as twenty-seven days old. (After His.) 



and the maxillary process. In front of the mandibular arch is a pentagonal 

 depression, termed the oral sinus or stomodosum, since it forms the future mouth. 

 It is bounded anteriorly by a median process, the fronto-nasal process, and laterally 

 by the maxillary processes (Fig. 714), and will be referred to again. 



These parts must now be considered with a little more detail. 



The fronto-nasal process covers the forebrain and contains the coalesced portion 

 of the trabeculge cranii ; it consists of a central or mid-frontal process and two 

 lateral parts. By the invagination 



of the olfactory pits, which COm- Cerebral hemisphere, 



municate below with the cavity of 



the mouth, each lateral portion is 



subdivided into an outer and an 



inner nasal process the latter 



having been termed by His the 



jirocessus globularis. The lateral 



nasal process is separated from the 



maxillary process by a groove which 



extends from the eye to the olfactory 



pit ; this is the rudiment of the 



lachrymal duct (Figs. 714, 715, and 



716). The globular processes are 



prolonged backward as plates, termed the nasal lamina ; these laminas are at first 



some distance apart, but, gradually approaching, they ultimately fuse and form the 



nasal septum, while the globular processes themselves meet in the middle line and form 



the prsemaxillae and central part of the upper lip (Fig. 717). The depressed part of 



74 



Fronto-nasal-^ 

 process. 



Stomodseitm.-- 



Lateral nasul process. 



Eye. 



"- Processus globularis. 

 Maxillary process. 



Mandibular arch. 

 Hyo-mandibular cleft. 



Fir,. 714. Under surface of the head of a human embryo, 

 about twenty-nine days old. (After His.) 



