HEAD SEGMENTS 



A varying number of somites behind the 6th grow downwards 

 and inwards to form the epibranchial and hypobranchial muscles, 

 the dorsal myotome dwindling away. A normal muscular segment, 

 serial with the trunk-myotomes, is generally not developed at all 

 in the head-region of a Gnathostome. 



The anterior mesoblastic somites extend downwards to form the 

 lateral plate, which passes between or is pierced by the gill-slits. 

 From this plate are developed the muscles of the visceral skeleton 



A. 



p 



pi 



t f a.A 



v. 



cb. adr. 



sc. 



Fig. 2. 



Diagrams of the segmentation of the anterior region of a Cyclostome, A. and a Gnathostome, 

 B. The skeleton is dotted : the myotomes, 1-13, are shaded « ith Lines (broken when vestigial) ; 

 the gill-slits (dotted) are shown below ; the dorsal root-nerves are black : the ventral root- 

 nerves are represented by dotted lines, a, auditory capsule; ad, auditory nerve; adr, 

 dorsal root of anterior spinal nerve; d.r, dorsal root ; eft, epibranchial nerve; /, facial nerve; 

 g, glossopharyngeal nerve; g.s, branchial slit, I-VII ; l.h, limit between head and trunk ; in, 

 'month; n, nostril; nt, notochord ; p, profundus nerve; pe, parachordal; s, spiracular slit; 

 sc, skeletal segment or scleromere ; t, trigeminal nerve ; tr, trabecula cranii : v, vagns m rve ; 

 v.t, ventral root. 



and alimentary canal, and the trapezius muscle passing to the 

 scapula. 



The cranial nerves, which used to be considered as radically 

 different from the spinal nerves, have now been recognised as, for 

 the most part, merely modified segmental nerves, some having 

 dorsal and others ventral roots. The old numbering of the cranial 

 nerves, founded on human anatomy, is often retained for conveni- 

 ence, but has been replaced by an enumeration referring to their 

 true metameric order. Those characters which distinguish them 

 from the spinal nerves are partly primitive and partly secondary. 



