21S EMBRYOLOGY OF THE LOWER VERTEBRATES - 11. 



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. 



HYPOBRANCHIAL 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 

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

 Lcpidosiren or Protopterus (Agar, 1907). 



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



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 c.h 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 be- 

 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. liy\ the branchial 

 arches being reduced in the fishes 

 in question. As the muscle 

 on with its development the an- 

 terior boundary of the portion belonging to myotorue 1 becomes 

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

 a similar intersection appears to demarcate y from z. 



I n oi her Vertebrates the hypobranchial or hypoglossal musculature 

 appears to originate in the same way difference occurring <>nl\ in 

 the number of niyotomes which take, part. Five appears to be 

 UK- most usual number (,SV //////////, doming: Teleosts, Harrison). 



KI.K- TKICAI. ORGANS. The conspicuous sign of a muscle becom- 

 ing active is that it- Changes its shape: an inconspicuous accomi>ani- 

 ment of this change of shape is tin- production of a slight electrical 

 disturbance In tin- case of most electrical organs we ha\e t<> do 

 with portion^ of the HIM dm in which the function of 



contraction ha> hem reduced to a subsidiary role >r abolished 



KK;. 11G. Dorsal view of anterior myo- 

 tomes of a Protopterus of stage 29. 

 (After Agar, 1907.) 



-.It. coracohyoid muscl<- ; .V, notochord 

 inuscl.'-bud to pectoral limb; ?/n, 



p.f, 



