596 GENERATION AND DEVELOPMENT. 



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

 four on each side (Fig. 230, A), and from or in connection with 

 these arches the following parts are developed : 



From the first arch, and its maxillary process, the superior 

 maxillary, the palate bone, and the internal pteryyoid plate of 

 the sphenoid bone, the incus and malleus and the lower jaw. 



FlG. 230. 



A, magnified view from before of the head and neck of a human embryo of about 

 three weeks (from Ecker) 1, anterior cerebral vesicle or cerebrum ; 2, middle ditto ; 

 3, middle or fronto-nasal process ; 4, superior maxillary process ; 5, eye ; 6, inferior 

 maxillary process, or first visceral arch, and below it the first cleft; 7, 8, 9, second, 

 third, and fourth arches and clefts. B, anterior view of the head of a human foatus of 

 about the fifth week (from Ecker, as before, Fig. IV). 1, 2, 3, 5, the same parts as 

 in A; 4, the external nasal or lateral frontal process; 6, the superior maxillary pro- 

 cess ; 7, the lower jaw ; X , the tongue ; 8, first branchial cleft becoming the meatus 

 auditorius externus. 



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

 from the so-called fronto-nasal process (A, 3, Fig. 230). From 

 the second arch are developed the stapes, the stapedius muscle, 

 the styloid process of the temporal bone, the stylo-hyoid liga- 

 ment, and the smaller cornu of the hyoid bone. From the third 

 visceral arch, the greater cornu and body of the hyoid bone. 

 In man and other mammalia the fourth visceral arch is indis- 

 tinct. 



Development of the Extremities. 



The extremities are developed in a uniform manner in all 

 vertebrate animals. They appear in the form of leaflike ele- 

 vations from the parietes of the trunk (see Fig. 231), at points 

 where more or less of an arch will be produced for them within. 

 The primitive form of the extremity is nearly the same in all 

 Vertebrata, whether it be destined for swimming, crawling, 

 walking, or flying. In the human foetus the fingers are at first 



