oh. v THE SKELETON 289 



Non - living material " secreted '' by cells consists no less of 

 modified cytoplasm, although here the cytoplasm so modified does not 

 form a continuous mass and retain its original position in regard to 

 the rest of the cell body. It arises commonly as isolated droplets or 

 particles which may secondarily run together within the body of the 

 cell or, without this happening to any obvious extent, are extruded 

 from it, passing on to a free cell surface or into the intercellular 

 matrix. The process can be followed by observation, naturally, only 

 in cases where the secretion runs together into discrete droplets 

 or particles sufficiently large to be visible under high powers of 

 the microscope, as commonly happens in gland-cells. Ear more 

 frequently the extruded particles are so small — possibly molecular — 

 i as completely to elude observation. Such is the case where the inter- 

 cellular substance undergoes skeletal modification : all that can be 

 observed is simply a gradual transformation in the physical and 

 chemical characters of the matrix, due in some cases to a gradual 

 change in the metabolic activities of the cells which inhabit it, in ' 

 others to the immigration into it of cell-colonists of a new type. 



The supporting skeleton of the Vertebrate is an endoskeleton ; 1 

 it is developed not on the outer surface of the body but within its 

 substance. In this it contrasts with the skeleton of the Arthropod 

 or Mollusc which is exoskeletal — consisting of thickened and stiffened 

 cuticle. In the case of the most ancient skeletal structure in the 

 Vertebrate body — the notochord — the stiff supporting character is 

 due to the individual cells being distended by fluid secreted in their 

 interior but as a rale in other skeletal tissues the stiffness is given 

 not by the cells but by the intercellular matrix. 



The Notochoed and its Sheaths. — A comparative study of the 

 Vertebrate skeleton shows that it illustrates three phases of evolu- 

 tionary progress (1) the notochordal phase, (2) the cartilaginous or 

 chondral phase and (3) the bony or osseous phase. Of these the 

 primitive is indisputably the first. It is a phase which is passed 

 through during ontogeny in all Vertebrates and it remains permanent 

 throughout life in Amphioxus. 



The notochord is in its origin a rod of cells split off from the 

 endoderm along its mid-dorsal line. This is seen in all the lower 

 Vertebrates. In some of the more primitive members of the group 

 the notochordal rudiment is for a time deeply grooved on its lower 

 side, so as to form an inverted gutter along the middle of the enteric 

 roof, and it may well be that this is to be regarded as the primitive 

 mode of formation of the organ. 



The notochord becomes constricted off as a cylindrical rod 

 extending along the dorsal side of the alimentary canal from a point 

 just behind the tip of the infundibulum to the tip of the tail. The 



1 It is regrettable that the term exoskeleton has crept into use by writers on 

 Vertebrate anatomy for structures such as fish-scales. As will be seen later these 

 are really endoskeletal, even the enamel being -developed on the rawer surface of the 

 epidermis. 



VOL. II U 



