45* 



STRUCTURE OF VERTEBRATA. 



S.L 



hairs it arises late and is always small. From the dermis are derived 

 the bony shields of armadillos, and a few related mammals, the bony 

 scutes of crocodiles and some other reptiles, and the scales of most 

 bony fishes. This again is readily explained by the fact that the 

 mesenchyme is also the skeletal layer of the embryo. The ordinary 



teeth of Vertebrates, as well as 

 the superficial or skin teeth of 

 gristly fishes, are largely formed 

 from the dermis, but are usually 

 covered by a thin coating of 

 ectodermic enamel. 



Muscular system. In all 

 Vertebrates the muscles of the 

 trunk arise from the primitive 

 segments or myotomes formed 

 in the embryo at the sides of the 

 nerve-cord. In Amphioxus and 

 Fishes the primitive segmented 

 condition of the muscles is 

 retained in the adults. Above 

 Fishes little trace of the seg- 

 mented condition persists in 

 the adult, except in the tail 

 region. 



The muscles of the limbs arise 

 in Elasmobranchs as buds from 

 the primitive segments ; buds 

 from several contiguous segments 

 grow into each fin. In most 

 other Vertebrates the formation 

 of the limb muscles is more 

 EC., Ectoderm ; S.c., spinal cord ; N., noto- complicated : they seem in some 

 chord : ao.. aorta ; s.d.. sesmental duct ; j j ^i r 



5? reproductive cell ; i.c.fbody cavity ; cases to _ arise independently of 

 P.C., segmentation cavity filled up with the primitive segments, 

 connective tissue; s.i.v., sub-intestinal Apart from the somatic 



vein; g., gut; c.v., cardinal vein; mt., musdes there are the visceral 

 myotome. , ,, , 



or mesenchymatous muscles 



which are derived from the 



ventral portion of the mesoderm. Unlike the striped voluntary 

 somatic muscles, they are unstriped except in special regions such as 

 the heart and the gullet. 



Skeletal system. Apart from the exoskeleton of skin- 

 teeth, scutes, shields, etc., the skeleton consists of the 

 following parts :- 



The skull and its associated " arches." 



FIG. 223. Transverse section through 

 an Elasmobranch embryo (diagram- 

 matic). After Ziegler. 



(u) Axial 



Skeleton. 



The backbone and associated ribs. 

 (The notochord is transitory except 

 in the simplest Vertebrates.) 



