4.VJ 



CHORDATA 



plate which has arisen from a fusion of dermal ossifications, becomes trans- 

 ferred to a deeper position, and contributes to the completion of the axial 

 skeleton. From what was said above about the relations of dermal and 

 true teeth it is readily seen that the lining of the mouth cavity is a 

 source of membrane bones. 



In describing the axial skeleton, the notochord comes first. This has 

 already been mentioned in connexion with lower Chordates. It persists 

 in the cyclostomes, but from them upwards it is gradually replaced by the 

 vertebrae arising around it. It is of entodermal origin (fig. 9), arising as a 



longitudinal band of the epithelium of the 

 archenteron (/, cli), and, becoming cut off, 

 comes to lie in the long axis of the body be- 

 tween digestive tract and nervous system (77, 

 777) ; here it forms a cyclindrical rod consisting 

 of a connective tissue which, as already said, 

 resembles plant tissues because of the vesicular 

 nature of its cells (fig. 39). 



In transverse section (fig. 511) the noto- 

 chord is surrounded by three layers, internally 

 by a fibrous notochordal sJieat/i, then an elastic 

 layer (not always present), the elastica externa, 

 so called because an elastica interna sometimes 

 occurs inside the noto'chordal sheath; and 

 lastly a skeletogenous layer (SS). This last 

 is a connective-tissue layer and is therefore 

 connected with the other connective tissues 

 which surround muscles, nerves, etc. It de- 

 serves special mention because in it the cartilages 

 and bones arise from which the vertebra? and 

 skull are formed. Cells from it can penetrate 

 the notochordal sheath, converting it into 

 fibrous cartilage, thus enabling it to participate 

 in the formation of the vertebrae. 



Since the notochord and its envelopes are elastic and give under the 

 strain of the muscles, they are unsegmented. The segmentation of the 

 axial skeleton begins with the appearance of firmer tissue as carti- 

 lage and bone. Then there is a separation of successive parts, and with 

 this the gradual formation of vertebral column and skull. In both 

 there is a connected series of modifications, whether studied onto- 

 genetically or comparatively, from the lower to the higher forms. 



The first parts of the vertebral column to appear are the upper 



FIG. 511. Transverse 

 section of axial skeleton of 

 Petromyzon (from Wieders- 

 heim). C, notochord; Cs, 

 notochordal sheath; Ee, 

 elastica externa; F, fatty 

 tissue; M, spinal cord; P, 

 its meninges; Ob, upper 

 process of skeletogenous 

 tissue; 55, skeletogenous 

 tissue; Ub, lower process of 

 (skeletogenous tissue.) 



