DEVELOPMENT. 793 



by a process which grows downward from the head. Between the 

 branches and the main first fold is the cavity of the mouth. The branches 

 represent the superior maxilla, and the main folds the mandible or lower 

 jaw. The central process, which grows down, is the fronto-nasal pro- 

 cess. 



In this way the so-called visceral arches and clefts are formed, four 

 on each side (fig. 486, A). 



From or in connection with these arches the following parts are devel- 

 oped : 



The first arch (mandibular) contains a cartilaginous rod (Meckel's 

 cartilage), around the distal end of which the lower jaw is developed, 

 while the malleus is ossified from the proximal end. 



When the maxillary processes on the two sides fail partially or com- 

 pletely to unite in the middle line, the well-known condition termed 

 cleft palate results. When the integument of the face presents a similar 

 deficiency, we have the deformity known as hare-lip. Though these two 



ILL 



Fig. 487. Embryo chick (4th day), viewed as a transparent object, lying on its left side 

 (magnified). C H, cerebral hemispheres ; F B, fore-brain or vesicle of third ventricle, with Pn. 

 pineal gland projecting from its summit ; M B, mid-brain ; C 6, cerebellum ; IV. V, fourth ven- 

 tricle; L, lens; c h s, choroidal slit: Cen. V, auditory vesicle; s m, superior maxillary process- 

 \F, 2F, etc., first, second, third, and fourth visceral folds; F, fifth nerve, sending one branch 

 (ophthalmic) to the eye, and another to the first visceral arch ; VII, seventh nerve, passing to 

 the second visceral arch; O. Ph, glosso-pharyngeal nerve, passing to the third visceral arch; 

 P g, pneumogastric nerve, passing toward the fourth visceral arch ; i v, investing mass ; c h, 

 notochord ; its front end cannot be seen in the living embryo, and it does not end as shown in 

 the figure, but takes a sudden bend downward, and then terminates in a point; Ht, heart seen 

 through the walls of the chest; M P, muscle-plates; TF, wing, showing commencing differentia- 

 tion of segments, corresponding to arm, forearm, and hand; 88, somatic stalk; Al. allantois; 

 H L, hind-limb, as yet a shapeless bud, showing no differentiation. Beneath it Is seen the 

 curved tail. (Foster and Balfour.) 



deformities frequently co-exist, they are by no means always necessarily 

 associated. 



The upper part of the face in the middle line is developed from the 

 so-called frontal-nasal process (A, 3, fig. 486.) From the second arch 



