460 



VERTEBRAL COLUMN OF MAMMALIA. 



The first vertebra to ossify is the second or third cervical, and the 

 ossification gradually extends to those behind. It does not commence 

 in the arches till somewhat later than in the bodies. For each arch 

 there are two centres of ossification, one on each side. 



The notochord persists for the greater part of foetal life and even 

 into post-foetal life. The larger vertebral portions are often the first 

 completely to vanish. They would seem in many cases at any rate 

 (Gegenbaur) to be converted into cartilage, and so form an integral 



part of the permanent vertebra?. Ru- 

 diments of the intervertebral portions 

 of the notochord may long be detected 

 in the ligameuta suspensoria. 



Schwarck (No. 420) states that in both 

 the intervertebral and the vertebral regions, 

 though less conspicuously in the former, 

 the cartilage is divided into two layers, an 

 inner and an outer. He holds that the 

 inner layer corresponds to the cartilaginous 

 notochordal sheath of the lower types, 

 and the outer to the arch tissue. Ossi- 

 fication (Gegenbaur) of the centra appears 

 in a special inner layer of cartilage, which 

 is probably the same as the inner layer 

 of the earlier stage, though this point has 

 not been definitely established. 



Mammalia. The early development 

 of the perichordal cartilaginous tube 

 and rudimentary neural arches is almost 

 the same in Mammals as in Birds. The differentiation into vertebral 

 and intervertebral regions is the same in both groups ; but instead of 

 becoming divided as in Reptilia and Birds into two segments attached 

 to two adjoining vertebra?, the intervertebral regions become in 

 Mammals wholly converted into the intervertebral ligaments (fig. 322 

 li). There are three centres of ossifications for each vertebra, two 

 in the arch and one in the centrum. 



The fate of the notochord is in important respects different from 

 that in Birds. It is first constricted in the centre of the vertebrce (figs. 

 320 E and 321) and disappears there shortly after the ossification ; 

 while in the intervertebral regions it remains relatively unconstricted 

 (figs. 320 E, 321 and 322 c) and after undergoing certain histological 

 changes remains through life as part of the nucleus pulposus in the 

 axis of the invertebral ligaments 1 . There is also a slight swelling of 

 the notochord near the two extremities of each vertebra (fig. 322 

 c and c"). In the persistent vertebral constriction of the notochord 



1 This view was first put forward by Lushka, and his surmises have been confirmed 

 by Kolliker and other ernbryologists. Leboucq (No. 424) however holds that the cells 

 of the notochord in the intervertebral regions fuse with those of the adjoining tissue ; 

 and Dursy and others deny that the nucleus pulposus is derived from the notochord. 



FIG. 321. LONGITUDINAL SEC- 

 TION THROUGH THE VERTEBRAL 

 COLUMN OP AN EIGHT WEEKS' HUMAN 

 EMBRYO IN THE THORACIC REGION. 



(From Kolliker.) 



v. cartilaginous vertebral body ; 

 U. intervertebral ligament ; ch. 

 notochord. 



