42t) 



STRUCTURE OF VERTEBRATA. 



ment of exoskeleton may perhaps be regarded as the 

 outcrop of a constitution which tends to extreme and 

 unprofitable passivity. 



The skin includes: — 



(n) The epidermis, usually in several layers, the outer "horny" 



stratum corneum, the inner " mucous " stratum malpighii, or 



mucosum ; both derived from the ectoderm or epiblast of the 



embryo. 

 (/') The dermis, cutis, corium, or under-skin, derived from the 



mesoderm or mesoblast of the embryo. 



From the epidermis are derived feathers, hairs, and some kinds of 

 scales. The dermis, as is natural when we consider its origin firom 

 the mesoblast (mesenchyme) or vascular 

 layer, assists in nourishing these epider- 

 mic structures. From the dermis are 

 derived the bony shields of armadillos 

 and a few related mammals, the bony 

 scutes of crocodiles and some other rep- 

 tiles, and the scales of most bony (Teleo- 

 stean) fishes. This again is readily ex- 

 plained 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. It 

 should be noted, however, that Klaatsch 

 has recently maintained the ectoderviic 

 origin of the skeleton forming cells 

 (scleroblasts) which form the scales of 

 Elasmobranchs 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, indeed, a growing tendency among 

 morphologists to strip the mesoderm of its 

 importance. It may be noted also that 

 Klaatsch ventures to suggest that the be- 

 ginning of skeleton in the ectoderm may 

 have something to do with excretion. 



JMusciiIar System. — In all Vertebrates 

 the muscles of the trunk arise from the primitive segments, or muscle 

 plates, found in the embryo at the sides of the ner\"e cord. In Aniphi- 

 oxtis and Fishes the primitive segmented condition of the muscles is 

 retained, as is seen in the myotomes visible externally in the lancelet. 

 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. 



I 



Fig. 137. — Transverse 

 section througli an Elasmo- 

 branch Embrj'O (diagram- 

 matic). (After ZlEGLER.) 



Ec, Ectoderm; S.c.^ spinal 

 cord ; X., noLochord ; ao., aorta ; 

 s.d., segmental duct; R., repro- 

 ductive cells ; S.C.. secondarj^ 

 coilome ; p.c.^ primary coalome 

 filled up with connective tissue ; 

 ^.z.r'., sub-intestinal \'ein ; g., gut; 

 C.I'., cardinal vein ; s.d., segmental 

 duct ; int., myotome. 



