. Outlines of zoology. Zoology. 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
. Outlines of zoology. Zoology. 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; , spinal cord; iv.,noto- , , c ,, ... chord; <w., aorta; , segmental duct; trunk anse from the primitive R., reproductive cells; , body cavity; segments, or muscle plates, , 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 tr
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Keywords: ., bookauthorth, bookcentury1800, bookdecade1890, booksubjectzoology
