324 THE 'IIK'K. 



THE DEVELOPMENT OF THE SKELETON. 



The skeleton of the chick, like that of the tadpole, consista 

 in its earliest stages of densely packed mesoblast cells, which 

 soon give rise, by the formation of an intercellular matrix, to the 

 primary or cartilaginous skeleton. This, in the adult, is almost 

 completely replaced by the secondary or bony skeleton, but 

 persists in places, especially in the skull. 



As in the frog, bone may appear either in direct relation 

 with the pre-existing cartilage, or independently of it, and 

 hence a distinction may be drawn between cartilage-bones and 

 membrane-bones ; the latter, which appear independently of the 

 cartilage, being almost confined to the head. 



1. The Vertebral Column. 



The vertebral column is formed from the ventral portions of 

 the mesoblastic somites or prot overt ebrae. After the muscle- 

 plates have separated off, the remaining or ventral portions of 

 the somites of each pair, which consist of indifferent and rather 

 loosely compacted mesoblast-cells, grow inwards towards the 

 median plane, and meet both above and below the spinal cord, 

 and below the notochord as well ; so that by the end of the 

 third day (Fig. 124) both the spinal cord and the notochord 

 have mesoblastic investments, which during the fourth day 

 increase considerably in thickness. 



Early on the fifth day, the transverse lines of demarcation 

 between the successive pairs of somites disappear, the mesoblast 

 becoming a continuous mass the whole length of the body, and 

 forming continuous investments to the notochord and the spinal 

 cord. This fusion only concerns the ventral portions of the 

 somites, the muscle-plates retaining their original distinctness. 



In the course of the fifth day, the mesoblast immediately 

 around the notochord becomes cartilaginous, forming a continu- 

 ous unsegmented cartilaginous tube, ensheathing the notochord 

 along the whole length of the body. At the sides of the spinal 

 cord, paired cartilaginous bars appear, which soon fuse with 

 the cartilaginous investment of the notochord, and become the 

 neural arches of the vertebrse. 



A little later, but before the close of the fifth day, further 

 histological changes occur in the cartilaginous tube surrounding 



