DEVELOPMENT OF THE PHARYNX AND NECK. 



31 



of mammals ; in fishes it disappears and real clefts are formed 

 between the arches. On the outer side of the membrane is the 



epiblast 

 hypoblast^ 



A 



mandib. arch 



1st cleft membrane 

 median lobe thyroid 

 2nd or hyoid arch 

 -(basis of pharyngeal 

 \ part of tongue 

 ceruical sinus 



oiglottis'; larynx* 'ijf//, 



Fig. 23.— Showing the Floor of the Pharynx of a 4th week human embryo. (After His.) 



" external cleft depression " : on the inner side, the " internal 

 cleft recess" (Fig. 23). From the hypoblastic lining of these cleft 

 recesses we shall see that the tonsil, thyroid and thymus arise ; 

 from 'the external depressions are formed the various branchial 



hypobk 



muscle 



cleft membrane 

 epiblast 

 nerve 

 cartilage 

 artery 

 uein 



nerue 



Fig. 24. — Schematic Section of a Visceral Arch. 



cysts and fistulae, which occasionally occur in the neck of the 

 adult. Each arch contains, as may be seen from figure 24 : 



(a) A skeletal basis of cartilage ; 



(6) An aortic arch ; 



(c) Vein ; 



(d) A larger nerve along its anterior border and a smaller 



along its posterior; 



(e) A muscle segment. 



