CHAPTER XIX. 



STRUCTURE AND DEVELOPMENT OF VERTEBRATES. 



To avoid the tiresome repetition of explanatory statements, 

 or burdening the description of fish or frog with general 

 questions of morphology, I have here summarised some of 

 the more important facts. 



As prominence has already been given to the essential 

 characteristics, — the dorsal and tubular nervous system, the 

 notochord, the gill-slits, the segments, etc., — we shall discuss 

 the various systems in their conventional order. 



Skin. — This includes — 



(a) The epidermis, usually in several layers, " horny," 



"mucous," etc., all derived from the ectoderm, 



or epiblast of the embryo. 



ip) 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 helps in nourishing them. From 

 the dermis are derived the bony shields of armadilloes and 

 a few related mammals, the bony scutes of crocodiles and 

 some other reptiles, and the scales of most bony (Teleostean) 

 fishes. Teeth and the skin teeth of gristly fishes are due to 

 both layers. 



Muscular System. — In the lancelet, in Fishes^ and in the 

 tail region of the higher Vertebrates, the muscles are disposed 

 in segments or myomeres. Above Fishes, the muscles of the 

 trunk no longer show in the adult any such arrangement. 



The muscles of the body-wall are derived from primitive 

 muscle-plates, and these from the original segments into 

 which the mesoblast of the embryo is divided. " From the 

 same source the muscles of the limbs are derived in Elasmo- 



