THE CCELOM AND THE SKELETON. 429 



DEVELOPMENT OF THE MUSCULAR SYSTEM. 



The majority of the body muscles are developed, as in the 

 chick, from the muscle plates of the protovertebras, or meso- 

 blastic somites. 



The great dorsal muscles of the neck and trunk, and the 

 muscles of the thoracic and abdominal walls, are derived directly 

 from the muscle plates, but the origin of many of the other 

 muscles is not determined with certainty. The muscles of the 

 head arise independently of the muscle plates ; and the muscles 

 of the limbs also arise independently, and in situ. It is pro- 

 bable, however, that in both these cases the mode of development 

 has undergone secondary modifications and abbreviations. 





DEVELOPMENT OF THE SKELETON 



There are in the rabbit, as in the chick or frog, three stages 

 in the development of the skeleton. The first, or earliest, stage 

 is that in which the notochord is the only specially skeletal 

 structure present ; the second stage is that in which a carti- 

 laginous skeleton is developed, not only in relation with the 

 notochord, but in the head and limbs as well ; while the third 

 or final stage is characterised by the development of bone, 

 which gradually becomes the dominant and essential constituent 

 of the skeleton. Bones arise either as cartilage-bones, in direct 

 connection with the cartilaginous skeleton ; or else independent^ 

 of this, as membrane-bones. 



It is important to remember that each of these stages is not 

 a further development of the preceding stage, but an indepen- 

 dently arising one, which displaces its predecessor. Thus the 

 cartilaginous skeleton does not arise from the notochord, but 

 outside this and independently of it, and gradually displaces 

 and obliterates it ; to be displaced in its turn by the bony 

 skeleton. So too the lower jaw is not formed from Meckel's 

 cartilage, but around it ; and the formation of the bone leads 

 ultimately to the obliteration of its cartilaginous predecessor. 



The development of the skeleton of the rabbit has not yet 

 been studied in detail, and there are many points on which our 

 knowledge is still very incomplete. 



