726 DEVELOPMENT OF THE VERTEBEAL COLUMX. 



of the bone elements appear to surround the notochord, that structure 

 does not itself, nor by its sheath, contribute to the formation of the 

 vertebral or basi-cranial bones, but merely lies within them ; and the 

 formative material, out of which the bones are produced, is derived 

 from mesoblastic substance which passes inwards from the primordial 

 vertebral plates, and envelopes the chorda external to its sheath. The 

 formation of the notochord, therefore, precedes that of the formative 

 bone-elements which afterwards envelope it, and the remains of the 

 notochord, unaffected directly by any ossifying change, are found in 

 the interior of the commencing bones, and may be traced even for a 

 long time throughout the whole length of the column of the bodies of 

 the vertebrre. 



This important fact was first demonstrated by H. Miiller, of Wm-tzbrn-g-, who 

 showed fui-ther that the notochord did not pass through the anterior arch of the 

 atlas, but was traceable directly from the body of the axis vertebra through its 

 odontoid process, and thence into the basi- occipital and basi-sphenoid bones, 

 reaching as far as the pituitary fossa. (Heinrich Miiller, " Ub. d. Vorkommen 

 von Eesten des Chorda Dorsalis b. Menschen nach der Geburt," in " Zeitsch. fiir 

 Rat. Med.," von Henle u. Pfeifer, 1858, b. ii. See also Gegenbaur, " Untersuch. 

 ub. Vergleich. Anat. Das Kopfskelet der Selachier," Leipzig, 1872. W. Miiller, 

 " Bau der Chorda DorsaUs," in '■ Jenasch. Zeitsch." b. vi. E. Dursy, " Zur Entwick. 

 des Kopfes," 1869, and Mihalkovics, '• On the Chorda and Pituitary Body," in 

 "Archiv filr Miki-oscop. Anat.," b. xi., 1875.) 



It may be mentioned further, as the result of H. Miiller's observations, that 

 though in general the chorda passes through the middle of the vertebral bodies, 

 the position was found subject to variation in the caudal portion of the column, 

 where it sometimes pa,?sed above, and at other times below, the vei-tebral bodies. 



The notochord itself has been generally held to be produced from an intruded 

 central column of mesoblastic cells, and this seems to be the nrode of origin in 

 birds ; but it may be doubtful whether it is the same in all animals. In sharks 

 Balfour finds that ihere is no median column derived from the mesoblast, and 

 attributes the origin of the notochord to the hj^joblast (Quart. Joum. of IMicro- 

 scop. Sc, Oct. 1874). The same origin is ascribed to it in mammals by Hensen, 

 who finds that the notochord is late of being formed in the rabbit, — an observa- 

 tion confii-med by KoUiker : Mihalkovics, on the other hand, is inclined to refer it 

 in all animals to the epiblast. However this may be, the tendency of recent 

 research appears to be to show that the notochord may be more nearly alUed 

 to epithelial structiu'es than to cartilage with which it has generally been 

 previously associated. It is at all events important to note that it is in many 

 respects different from the parts ascertained to proceed from the mesoblast, that 

 it never combines with their elements, and that there is no penetration of its 

 substance by connective tissue or blood-vessels, as happens in all other parts 

 derived from the mesoblast. 



The interesting observations of Kowalevsky on the existence of a chorda 

 dorsalis in Ascidia (Mem. de TAcad. de St. Petersboiu'g, torn. x. and xi., 1867 and 

 1868), would appear to show that this structure, and the tj^je of development 

 which accompanies it, are not confined to vertebrate animals, and that in them 

 the notochord may present more of a merely vestigial character than constitute 

 an important element in the formation of the skeleton. The constancy of its 

 position and relations, however, is an important fact regai-ding its history. 



The notochord does not undergo transverse segmentation in the same 

 manner as the protovertebral plate does. It remains undivided to the 

 last, but in the course of vertebral ossification it shows alternate dimi- 

 nutions and enlargements of its diameter, corresponding in number and 

 position with the vertebral divisions. One of these enlargements is 

 found between the odontoid process and the basi-occipital bone, and 



