208 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



E. Miiller (1911) believes the connecting bridges in the case of 

 Acanthias to be special developments of a syncytial network which 

 lies between the buds from the commencement : he fails to find in 

 this animal any evidence of shifting of muscle-cells along the bridges. 

 The matter appears to stand in need of further investigation. 



Already within the group of Elasmobranchs we find modification 

 of the typical mode of muscularization of the fins outlined above. 

 In the case of- the most anteriorly placed muscle-bud of the pectoral 

 fin of Spinax the bud resolves itself into its constituent cells which 

 separate before giving rise to muscle-cells. Again at the anterior 

 and posterior limits of the pectoral limb musculature in Pristiurus 

 and Torpedo the compact stage of the muscle-bud is eliminated 

 entirely and the cells which muscularize the fin are budded off 



ot 



FIG. 115. Diagram to illustrate the arrangement of mesoderm segments in the head- 

 regiou of a young Elasmobranch embryo. (From a drawing by Agar.) 



>t, otoc-yst ; *jil'-, splanchnocoele ; t, u, v, w, occipital myotomes ; 1, 2, 3, anterior myotomes ; 

 I, II, etc., visceral clefts; *, "Fourth" myotomt- of van Wi.jhc. 



separately from the inyotome, wandering from their place of origin 

 into the limb rudiment and there settling down (Braus). 



Amongst Vertebrates outside the group of Elasmobranchs 

 such modification appears to be the rule. Thus in Acipenser and 

 apparently in Lacerta typical muscle-buds arise singly from the 

 myotomes concerned. In Teleosts Harrison finds muscle-buds in the 

 pelvic fin but a diffused origin in the pectoral. In Lung-fishes and 

 Amphibians the origin seems to be again diffuse and the same 

 appears to be the case in Birds. 



MESODERM OF THE HEAD-REGION. There are two important. 

 characteristics "f tlm head -region of the Vertebrate ultimately 

 connected with the muscular system: (1) loss of Ilex i I >ility, associated 

 with the evolution of brain and skull and ('_') special muscularity of 

 the wall of tin; alimentary canal, associated \\iih the presence of 

 important movable skeletal >i met ures enclosed in the substance 

 of the visceral arches. These peculiarities find their expression 

 (1) in the tendency in Mippre.s>ion of the myotomes of the head- 



