174 Vertebrate Embryology 



probably derived from the spinal nervous sys- 

 tem, and is hence merely a specialized part 

 of that system. According to this view it 

 originates as a series of outgrowths from the 

 spinal ganglia, which outgrowths extend in- 

 ward until they nearly reach the dorsal aorta : 

 at their inner ends they become enlarged to 

 form the sympathetic ganglia, and these ganglia 

 send out processes which fuse to form the 

 longitudinal commissures. 



The Development of the Sense Organs 



The development of the sense organs, the 

 eye, ear, and nose, though, in most respects, 

 similar to the development of the same organs 

 in the frog, will be described in more detail 

 than was done in connection with the frog. 



The eye. — Although it begins in the early 

 part of the second day and is not completed 

 until late in the period of incubation, it will be 

 most convenient to describe the entire develop- 

 ment of the eye at this point, rather than wait 

 until the development of the following days is 

 discussed. In any case, the greater part of the 

 changes here described take place before the 

 end of the third day. 



