212 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



N 



functional muscles derived from them are connected with the eye- 

 ball an organ which becomes complete and functional only at a 

 relatively late period of development. 



Hypobeanchial or Hypoglossal Musculature. — In addition 

 to the musculature already indicated the Vertebrate head possesses 

 on its ventral side a system of hypobranchial muscles which have 

 the appearance of a prolongation forwards of the longitudinal 

 muscles of the ventral body-wall. This hypobranchial musculature 

 as a matter of fact does arise in ontogeny as a prolongation forwards j 

 of the anterior trunk and occipital myotomes, as is well shown by'i 

 Lepidosiren or Protopterus (Agar, 1907). 



About stage 29 the ventral ends of myotomes y, z and 1 are seen 



j to be growing out at their ventral 

 ends into a long slender prolonga- 

 tion (Fig. 116). These processes 

 grow outwards in front of the 

 pronephros and undergo complete 

 fusion at their tips. The fused 

 apical portion ch soon separates - 

 from the parent myotomes and 

 grows forwards, on each side of 

 the pericardiac cavity, until it 

 reaches the hyoid arch. It now 

 spreads ventrally until it meets its 

 fellow below the pericardiac cavity. 

 The common mass so formed he- 

 comes converted into a sheet of 

 longitudinal muscle-fibres, attached 

 posteriorly to the shoulder girdle 

 and anteriorly for the most part to 

 the hyoid arch (coracohyoid muscle, \ 

 Fig. 117, cor. hy), the "branchial 

 arches being reduced in the fishes 

 in question. As the muscle goes 

 on with its development the an- 

 terior boundary of the portion belonging to myotome 1 becomes 

 marked by a connective-tissue intersection, while in some specimens 

 a similar intersection appears to demarcate y from z. 



In other Vertebrates the hypobranchial or hypoglossal musculature 

 appears to originate in the same way — difference occurring only in 

 the number of myotomes which take part. Five appears to be 

 the most usual number (Scy Ilium, Corning i Teleosts, Harrison). 



Electrical Organs.— The conspicuous sign of a muscle becom- 

 ing active is that it changes its shape : an inconspicuous accompani- 

 ment of this change of shape is the production of a slight electrical 

 disturbance. In the case of most electrical organs we have to do 

 with portions of the muscular system in which the function of 

 contraction has been reduced to a subsidiary role or abolished 



Pig. 116. — Dorsal view of anterior myO' 

 tomes of a Protopterus of stage 29 

 (After Agar, 1907.) 



ch, coracohyoid muscle ; N, notoehord ; p.f, 

 muscle-bud to pectoral limb ; pn, pronephros. 



