54 



A MANUAL OF ANATOMY 



where it is continuous with the wall of the neurenteric canal. On either side 

 of it there are the mesodermic somites. 



The notochord is of temporary duration, and a considerable part of it is 

 replaced by the bodies of the vertebrae. Certain 

 vestiges of it, however, persist throughout life, these 

 being represented by the central pulp of the inter- 

 vertebral discs. 



Development. — The notochord is developed from 

 the entoderm. Its development may be considered 

 under three stages — namely, (i) the formation of 

 the notochord al plate, (2) the conversion of this 

 plate into the notochordal tube, and (3) the solidifi- 

 cation of this tube to form the notochord. 



First Stage. — The cells of the entoderm along the 

 dorsal or upper wall of the archenteron (beneath 

 the neural tube) become thickened, and so form 

 the notochordal plate. 



Second Stage. — (i) The notochordal plate be- 

 comes curved; (2) its lateral margins incline 

 ventrally or downwards; and (3) they turn inwards 

 and meet at the median line on the ventral aspect 

 of the notochordal plate. In this manner the 

 notochordal tube is formed. 



Third Stage. — -The notochordal tube is at first 

 hollow. The cells of its walls, however, are pro- 

 liferous, and by an increase in their number the lumen of the tube becomes 

 occluded, and the fully-developed notochord is formed as a solid cylindrical rod. 



'■■ m Odontoid Process 



Notochord passing 



through Primitive 



Body 



Swelling between 



two Primitive 



Bodies 



Fig. 37. — The Noto- 

 chord (Cervical 

 Region). 



Development of the Vertebral Column. 



The notochord forms the axis round which the vertebral column is developed, 

 and the formation of the notochord may be regarded as constituting the 

 first or notochordal stage. The notochord and the neural tube, which lies 

 along its dorsal aspect, become surrounded by mesenchyme (mesoderm), 

 and this undergoes chondrification and subsequently ossification. As the 

 process of ossification proceeds, a great part of the notochord becomes con- 

 stricted at regular intervals, where the bodies of the vertebrae are undergoing 

 ossification, and these portions of it ultimately disappear. The parts of it, 

 however, round which the intervertebral discs are formed persist and con- 

 stitute the central pulp of each disc, as stated. 



In addition to the notochordal stage of development, there are three other 

 stages — namely, membranous, cartilaginous, and osseous. 



Membranous Stage. — The notochord and neural tube become surrounded, 

 as stated, by a mesenchyme, which forms a membranous sheath, spoken of 

 as the skeletogenous sheath. This sheath constitutes the primitive membranous 

 vertebral column. It is derived from the sclerotomes of the mesodermic 

 somites which lie along either side of the notochord. Each sclerotome 

 is the innermost segment of a somite, and the successive sclerotomes he 

 serially on either side of the notochord and close to it. They are formed of 

 mesenchyme, and their cells multiply very rapidly. As they do so, they 

 extend ventralwards, dorsalwards, cephalicwards, caudalwards, and mesially, 

 between the notochord and the neural tube. In this manner the notochord 

 becomes surrounded by a sheath. On either side this sheath furnishes two 

 prolongations, which pass dorsalwards, one on either side of the neural tube, 

 and meet over its dorsal aspect. There are thus formed continuous sheaths 

 for the notochord and neural tube, and so the chordal or skeletogenous 

 sheath is formed. The cells of the sclerotomes also extend in an outward 

 direction, and these lateral extensions he between, and separate, contiguous 

 muscle-segments, each of these being a segment of a mesodermic somite, in 

 which it lies external to the sclerotome. These lateral extensions are called 

 the intersegmental septa, or li^amenta intermuscularia. 



