400 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



The development of the cranial and spinal 

 nerves in Mammals is as far as is known essentially 

 the same as in the chick, for an account of which see 

 p. 123 et seq. 



Sympathetic nervous system. The development 

 of, the sympathetic system of both Aves and Mammalia 

 has not been thoroughly worked out. There is how- 

 ever but little doubt that in Mammalia the main por- 

 tion arises in continuity with the posterior spinal 

 ganglia. 



The later history of the sympathetic system is inti- 

 mately bound up with that of the so-called supra-renal 

 bodies, the medullary part of which is, as we shall see 

 below, derived from the peripheral part of the sympa- 

 thetic system. 



THE ORGANS DERIVED FROM MESOBLAST. 



The vertebral column. The early development of 

 the perichordal cartilaginous tube and rudimentary 

 neural arches is almost the same in Mammals as ia 

 Birds. The differentiation into vertebral and inter- 

 vertebral regions is the same in both groups; but instead 

 of becoming divided as in Birds into two segments 

 attached to two adjoining vertebrae, the intervertebral 

 regions become in Mammals wholly converted into the 

 intervertebral ligaments (Fig. 135 li). There are three 

 centres of ossification 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 centres of the vertebrae (Fig. 134) and disappears 

 there shortly after the beginning of ossification ; while in 



