32 



HUMAN EMBRYOLOGY AND MORPHOLOGY. 



Formation of the Cervical Sinus. — The first arch especially, 

 and also the second, grow and increase at a much greater rate 

 than the third and fourth. It is observed that the second arch 

 (hyoid) which in fishes forms the operculum for the gills, grows 

 over and buries the third and fourth. As it covers them over and 

 comes in contact with the body wall behind the fifth arch (see A 

 and B, Fig. 23), the epiblast covering the third and fourth arches 

 and clefts is buried. The epiblast so enclosed forms the lining of 

 the cervical sinus. It usually disappears, but may remain and form 

 a cyst in the neck, which opens some distance above the sterno- 

 clavicular joint. If the outer cleft depression in front of or 

 behind the third arch persist, it must open in the cervical sinus. 



What becomes of the Visceral Clefts. — By the end of the 

 second month the clefts, or, to be more exact, the representatives 



mandible (1st arch) 



yoid (2nd & 2rd arches) 

 sterno-mastoid 



cervical sinus 

 3rd & 4th clefts 



Fib. 25— Showing the position of the External Cleft Depressions in the Adult. 



of clefts in the human embryo, have 1 disappeared, except the upper 

 part of the first which forms : 



