844 



DEVELOPMENT 



[CH. LIX. 



The rudiments from which the axial skeleton of the body is 

 formed are the protovertebrae or mesoblastic somites (see p. 833). 

 Each protovertebra separates into three parts: 1. An outer, the 

 cutaneous lamella, from which the deeper parts of the skin and the 

 subcutaneous tissues of the body are developed. 2. A middle portion, 

 the muscle plate. From the muscle plates all the striped muscles of 

 the body, with the exception of those of the heart, are formed. 



8 



FIG. 638. Diagram of a transverse section of an ovum showing differentiation of protovertebrae and 

 formation of amnion folds, primitive intestine, and yolk-sac. 



10. Primitive intestine. 



11. Co2lom (extra-embryonic). 



12. Coelom (intra-embryonic). 



14. Primitive dorsal blood-vessel. 



15. Scleratogenous part of protovertebra. 



6. Splanchnic mesoblastl , ^immi.nrmioiir 16 - Muscle plate part of protovertebra. 



7. Hypoblast J IS. Splaiichnopleur. 17. Cutaneous lamella of protovertebra. 



8. Yolk-sac. 18. Amnion folds. 



1. Spinal cord. 



2. Notochord. 



3. Amnion cavity. 



3. A scleratogenous segment. The scleratogenous segments fuse 

 together round the neural tube and the notochord, and in this way a 

 continuous membranous vertebral column is formed. This is cleft on 

 each side in every segment, for the passage of the nerve-roots and the 

 accompanying blood-vessels. Part of the membranous column is 

 converted first into cartilaginous and afterwards into bony vertebrae ; 

 other parts are transformed into inter vertebral discs and ligaments, 

 and the remainder forms the membranes which line the spinal canal 

 and surround the spinal cord. From the sides of the vertebrae the 



