828 



DEVELOPMENT. 



[CH. LVIII. 



eyes are formed by depressions of the skin which meet the 

 optic vesicles, outgrowths from the brain. The nasal pits take 

 origin as two simple depressions, the primary olfactory or nasal 

 pits ; these become connected to the first cleft or mouth. The 

 second slit, which corresponds to the spiracle of fishes, becomes 

 the external auditory meatus and the Eiistachian tube. The 

 remainder, which correspond to the gill slits of fishes, entirely close 

 up in mammals, and no gills are developed on their margins. 



The anterior border of each cleft forms a fold or lip, the 

 branchial or visceral fold. The posterior border of the last 

 cleft is also formed into a fold, so that there are four clefts and 

 five folds, but the three most anterior are far more prominent 

 than the others, and of these the second is the most conspicuous. 

 The first fold nearly meets its fellow in the middle line, the second 

 less nearly, and the others in order still less so. The first arch 



Fig. 644. A. Magnified view from before of the head and neck of a human embryo of 

 about .three weeks (from Ecker). i, 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 

 post-oral cleft ; 7, 8, 9, second, third, and fourth arches and clefts. B. Anterior 

 view of the head of a human foetus of about the fifth week (from Ecker). i, 2, 3, 5. 

 the same parts as in A ; 4, the external nasal or lateral frontal process ; 6, the 

 superior maxillary process ; 7, the lower jaw ; x , the tongue ; 8, first post-oral cleft 

 becoming the meatus auditorius externus. 



gives off a branch from its front edge, which passes forwards to 

 meet its fellow, but these offshoots do not quite meet, being 

 separated by a process which grows downwards from the head. 

 Between the branches, or maxillary processes, 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 process. 



From or in connection with these arches the following parts are 

 developed : 



The first arch (mandibular) contains a cartilaginous rod 



