516 



C HOED AT A. 



it is gradually replaced by the vertebrae arising around it. It is of 

 'cntodermal origin (fig. 9), arising as a longitudinal band of the 

 epithelium of the archenteron (/, di), and, becoming cut off, 

 comes to lie in the long axis of the body between digestive tract 

 and nervous system (//, ///) ; here it forms a cylindrical rod con- 

 sisting of a connective tissue which, as already said, resembles 

 plant tissues because of the vesicular nature of its cells (fig. 38). 

 In transverse section (fig. 555) the chorda is surrounded by 

 three layers, internally by a fibrous noto- 

 chorda] sheath, then an elastic layer (not 

 always present), the elastica externa, so 

 called because an elastica interna is some- 

 times present inside the notochordal 

 sheath; and lastly a skeletogenous layer 

 (SS), also called the outer notochordal 

 sheath. This last is a mesodermal con- 

 nective-tissue layer and is therefore con- 

 nected with the other connective-tissue 

 sheaths which surround muscles, nerves, 

 etc., and deserves special mention because 

 in it the cartilages and bones arise from 

 which the vertebrae and skull are formed. 

 Cells from it can penetrate the notochor- 

 dal sheath, converting it into fibrous car- 



, ._ 



tilage, thus enabling it to participate in 



, to ' 



the formation of the vertebras. 



. ,n , n -, -. ., 1,1 



Since the notochord and its sheaths 

 are elastic and give under the strain of the 

 muscles, they are unsegmented. The seg- 

 mentation of the axial skeleton begins with 

 the appearance of firmer tissue in cartilage and bone. Then there 

 is a separation of successive parts, and with this the gradual forma- 

 tion of vertebral column and skull. For both there is a con- 

 nected series of developments, if studied with reference to the 

 ontogenetic processes or in the comparative manner from the 

 lower to the higher forms. 



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

 and lower (figs. 555, 556), or neural and hcemal arches. These 

 consist of paired parts in the skeletogenous layer which abut 

 against the notochord, and which are usually a pair to the somite, 

 although occasionally two or more pairs, the arches proper and 

 the intercalaria^ may occur. The neural arches (arcus vertebras 



/z 



555.-Transverse section 



of axial skeleton of Pe- 



tromyzou. (From wieders- 



heim.) C, notochord; Cs, 



notochordal sheath ; Ee, 



elastica externa : f, fatty 



tissue M, spinal cord ; P, 



tissue ; -SVS, skeletogenous 

 tissue ; Ul>, lower process 

 of skeletogenous tissue. 



