DEVELOPMENT OF THE DOG-FISH. . 45 



mouth communicates with the brain. The trabeculse then fuse 

 with one another and with the parachordals, and thus cut off the 

 upper part of the mouth, which is transformed into part of the 

 pituitary body. A cartilaginous floor is thus formed beneath brain 

 floor, grows outwards and upwards, fusing with the nasal and ear 

 capsules but not with eye capsules, and tries to roof in head, and 

 so form a -complete cartilaginous box. It never, however, com- 

 pletely roofs it in, one or two spaces being covered by the skin. 



VISCERAL ARCHES AND CLEFTS. 



Below and on the sides of cranial parts of head,'a series of pro- 

 cesses or bars of cartilage are developed in pairs. The tissue 

 between these bars become absorbed, forming clefts, which remain 

 open for a longer or shorter period. The first cleft lies between 

 the first and second arch, so that there is one more arch than cleft. 



There are seven branchial arches and six clefts. Cartilage 

 developed in first arch is called Meckel's cartilage. From near 

 its proximal end another bar of cartilage is given off, which is 

 called the pterygo palatine. This forms the upper jaw, while 

 Meckel's itself forms the lower jaw. Between the two the mouth 

 is formed. From proximal end of Meckel's cartilage another 

 small cartilage is formed — the spiracular or meta-pterygoid- 1 — 

 which ossifies as the malleus in man. The second arch is called 

 hyoid, and has three pieces — (1) Hyomandibular, which connects 

 jaws with ear capsule (connected to hyoid by a ligament). (2) 

 Basihyal, which goes to support tongue. (3) Hyoid piece itself. 



Other five simply form branchial or gill arches. 



The limbs are believed to be formed from arches called extra 

 branchial arches. There are in addition five pairs of labial 

 cartilages : — First pair extending over nasal capsules ; second and 

 third pairs above nasal aperture ; fourth pair in connection with 

 upper jaw ; fifth pair with lower jaw. 



OLFACTORY ORGANS. 



Two pits, one at each side, appear in primitive head. These 

 deepen, and their lining cells become modified to receive sense 



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