43° 



STRUCTURE OF VERTEBRATA. 



mesoblast (mesenchyme) or vascular layer, assists in nourishing these 

 epidermic structures. In the case of feathers and the scales of Rep- 

 tiles, this dermic papilla is of primary importance, but in the case of 

 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 Teleostean 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 coat- 

 ing of ectodermic enamel. It 

 should be noted, however, that 

 Klaatsch has recently maintained 

 the ectodermic origin of the skele- 

 ton-forming cells (scleroblasts), 

 which form the scales of Elas- 

 mobranchs and Teleosteans, and 

 that there are hints in higher 

 forms that the ectoderm has more 

 to do with the skeleton than is 

 usually allowed. There is, in- 

 deed, a growing tendency among 

 morphologists to strip the meso- 

 derm of its importance. It may 

 be noted also that Klaatsch 

 ventures to suggest that the 

 beginning of skeleton in the 

 Fig. 184.— Transverse section through ectoderm may have something 

 an Elasmobranch embryo (diagram- to do with excretion, 

 matic).— After Ziegler. Muscular system.— In all 



„ _ . . ,, . , . ,, Vertebrates the muscles of the 



Ec, Ectoderm; 6.c, spinal cord ; iv.,noto- , , c ,, ... 



chord; <w., aorta; s.d., segmental duct; trunk anse from the primitive 



R ., reproductive cells ; s.c , body cavity ; segments, or muscle plates, 



p.c, segmentation cavity filled up with formed in the embryo at the 



sides ot the nerve-cord. In 

 Amphioxus and Fishes the 

 primitive segmented condition 

 of the muscles is retained, as is 

 seen in the myotomes visible externally in the former. Above Fishes 

 little trace of the segmented condition persists in the adult, except in the 

 tail region. The muscles of the head arise from the primitive segments 

 of that 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 complicated ; they seem in some cases to arise independently of 

 the primitive segments. 



Most of the visceral muscles consist of unstriped fibres, but those of 



connective tissue 

 vein ; g., gut ; c.v. 

 myotome. 



s.i.v., sub-intestinal 

 , cardinal vein; mt., 



