VERTEBRATE SKELETON 



INTRODUCTION 



As included here, the skeleton of Vertebrates embraces all 

 harder protective and supporting structures derived wholly or in 

 part from the mesenchyme, one of the two divisions of the mesoderm. 

 This definition excludes such protective features as hair, feathers, 

 scales of snakes and lizards, and the like, which are formed by the 

 ectoderm, also some, the same in origin (hoofs of mammals, beaks of 

 birds) which are firmer than many true skeletal parts. To these 

 mesenchymatous parts must be added the notochord, derived from 

 the entoderm.^ 



Embryology. — As implied above any part of the mesenchyme is 

 a potential source of skeletal parts, hence it is necessary to know the 

 distribution of the layer, especially in the embryo, to follow the 

 course of skeletal formation. So there follows a brief and very 

 generalized resume of some features of embryology. In Vertebrates, 

 gastrulation and the early differentiation of the germ-layers, is a 



complex process (especially so in Amniotes) and details are not 



■» 



' The notochord, characteristic of all Chordata, occurs on no invertebrate. It 

 extends from the tip of the tail forwards, in 

 Vertebrates, to the hypophyseo-infundibular 

 region, a little behind the tip of the head. It 

 forms an a.xis around which the vertebrae and 

 part of the skull develop. More or less of it 

 persists through life in the lower groups, but 

 it almost wholly disappears from Amniotes 

 before the adult stage is reached. In lower 

 Vertebrates and in Tunicates and Amphioxus 

 it is clearly a product of the entoderm (fig. i), 

 being cut off from the mid-dorsal line of the 

 archenteric wall. The fusion of germ layers 

 is so complete in Amniotes in the early stages 

 students have derived the notochord 



that 



Fig. I. — Section of Acanthias embryo 

 before formation of somites, showing 

 notochordal cells as part of entoderm. 

 a, archenteron; c, notochordal cells; ec, 

 ectoderm; m, forming mesothelium; n, 

 neural groove; y, yolk. 



from each of the three layers. If this struc 



ture be the same in all Vertebrates, its origin 



must be the same in all, otherwise ther^ is an enormous difficulty in reconciling its 



homology in lower and higher groups. 



1 I 



