360 REPRODUCTION 



From them also are developed the muscles and skin of the 

 back. 



The cranium is developed as a modification of the vertebral 

 column. 



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

 ous. 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 sub- 

 divided later. 



Nervous System. The origin of the nervous system has been 

 indicated in describing the neural canal. The mesodermic 

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

 cord is left. Headward the neural canal terminates in a dilated 

 extremity, which soon becomes divided into three vesicles, an- 

 terior, 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 predomin- 

 ant size of the cerebrum. At first there are no cerebral convolu- 

 tions, but later the cavity of the cranium seems too small for the 

 brain and the characteristic 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 auditory 

 vesicle from the posterior brain vesicle. 



The alimentary canal is formed by being pinched off from 

 the mesodermic layer of splanchnopleure. It communicates for 

 some time by means of the vitelline duct with the umbilical 

 vesicle. When cut off from the latter it is a straight tube, occupy- 

 ing the long axis of the body just in front of the vertebral column, 

 and is divided into the foregut, hindgut and a central part. 

 Later it communicates above with the pharynx and mouth and 

 opens below upon the external body surface (anus). The liver 



