NERVOUS SYSTEM 361 



itself become those bodies. The protovertebrse were seen to 

 lie longitudinally on either side of the notochord. These 

 grow around the neural canal dorsally and the notochord 

 ventrally to form the vertebrae. From them also are de- 

 veloped the muscles and skin of the back. 



The cranium is developed as a modification of the verte- 

 bral column. 



All the bones are in early fetal life cartilaginous or mem- 

 branous. Centers of ossification appear at one or more 

 points in each bone. 



The bones of the extremities are not at first separate. 

 They bud out from the upper and lower parts of the trunk, 

 to be subdivided later. 



Nervous System. The origin of the nervous system has 

 been indicated in describing the neural canal. The meso- 

 dermic cells multiply and fill the tube, until only the canal of 

 the spinal cord is left. Headward the neural canal termin- 

 ates in a dilated extremity, which soon becomes divided into 

 three vesicles, anterior, middle and posterior. From these 

 are developed the different parts of brain. Some of these 

 parts develop much more rapidly than others, and we thus 

 account for the predominant size of the cerebrum. At first 

 there are no cerebral convolutions, but later the cavity of 

 the cranium seems too small for the brain and the charac- 

 teristic infoldings occur. 



The eye is formed by the projection of the optic vesicle 

 from the side of the anterior brain vesicle. 



The internal ear is formed by the projection of the audi- 

 tory vesicle from the posterior brain vesicle. 



The alimentary canal is formed by being pinched off 

 from the mesodermic layer of splanchnopleure. It com- 

 municates for some time by means of the vitelline duct with 

 the umbilical vesicle. When cut off from the latter it is a 

 straight tube, occupying the long axis of the body just in 

 front of the vertebral column, and is divided into the fore- 

 gut, hindgut and a central part. Later it communicates above 



