Branchial Clefts 



123 



A NOTE UPON DEVELOPMENT GENERALLY 



From the external layer of the blastoderm (/SXaoros-, germ ; 

 iri)\he epiblast the entire nervous system, central and peripheral, 

 is developed, as are, also, the organs of sense, the cuticular covering 

 of the body and the lining of the mouth, together with its accesspry 

 glands. 



From the hypoblast are developed the epithelium of the alimentary 

 canal and air-passages. 



From the mesoblast come the bones, muscles, and vessels ; the 



A, Embryo at three weeks ; i and 2, 

 cerebrum ; 3, fronto-nasal process ; 

 4, superior maxillary plate ; 5, eye ; 6, 

 inferior maxillary, or mandibular 

 plate ; 7, 8, and 9, second, third, and 

 fourth plates, or branchial arches, and 

 below each plate the corresponding 

 pharyngeal cleft. AA, i, 2. 3, and 5, 

 same as in A ; 4, lateral frontal or ex- 

 ternal nasal plate ; 6, sup. max. plate ; 

 7, mandibular ; 8, first pharyngeal 

 cleft, which becomes auditory passage ; 

 above and below 6 are seen the orbital 

 and mandibular fissures respectively. 



B. i, Lower jaw; i', first post-oral cleft widening 

 out to form ext auditory meatus ; the second 

 cleft is still visible, but the third and fourth 

 clefts have become effaced, c, foetus at nine 

 weeks ; first pharyngeal cleft is now obliterated, 

 and the pinna, i', is beginning to grow up 

 around the unclosed dorsal end. 



skin (not the epidermis) ; the alimentary canal (not the epithelium), and 

 the genito-urinary apparatus. 



The facial part of the head is developed from bar-like growths from 

 the cranial base, some in front of, and some behind, the buccal cavity ; 

 the mouth, which is at first closed in, being a cleft between the facial 

 plates. The pre-oral plates are the median fronto-nasal (p. 105) and 

 the pairs of the lateral nasal and maxillary plates. The plates 

 behind the mouth (post-oral] are in five lateral pairs : the mandibular, 

 for the lower jaw ; the hyoid, for the upper part of the hyoid bone ; and 

 three pairs down the neck. The post-oral plates are sometimes 

 called branchial (flpny^ta, gills) from their corresponding to the gill- 

 plates of aquatic vertebrates. 



The branchial clefts are the slits below the branchial plates, or the 



