OH. LVlli.] VISCERAL CLEFTS AND ARCHES. 829 



(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 

 completely 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 



m 



Fig. 645. 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 I'n, pineal gland projecting from its summit ; M B, mid-brain ; C b, cerebellum ; 

 IV, V, fourth ventricle ; //, lens ; e h s, choroidal slit ; Cen. V, auditory vesicle ; * TO, 

 superior maxillary process; i /'. . /", &c., first, second, third, and fourth visceral folds ; 

 I '. 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 ; G. fh, glosso- 

 pharyngeal nerve, panning to the third visceral arch ; /'.</. pneumogastric nerve, pass- 

 ing towards the fourth visceral arch ; t r, 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 

 lint takes a sudden bend downwards, and then terminates in a point ; //'. heart seen 

 through the walls of the cheat ; M I', muscle-plates : H', wing, showing commencing 

 differentiation of segments, corresponding to arm, forearm, and hand ; .S' >'. somatic 

 talk ; Al, allantois ; // /,, hind-limb, as yet a shapeless bud, showing no differentia- 

 tion. Beneath it is seen the curved tail. (Foster and Balfour.) 



hare-lip. Though these two 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 frontal-nasal process (A, 3, fig. 644). 



From the second arch are developed the incus, stapes,* and 

 stapedius muscle, the styloid process of the temporal bone, the 

 stylo-hyoid ligament, and the smaller cornu of the hyoid bone. 



* The origin of the ear ossicles given in the text is only one of five or six 

 different views that have been advanced bv different observers. 



