40 Vertebrate Embryology 



The olfactory lobe is formed by the fusion 

 of an outgrowth from the anterior end of 

 each cerebral hemisphere. The cerebral hem- 

 ispheres are at first unpaired and later become 

 double, while the olfactory lobe is at first 

 double and afterwards becomes single. 



There are many details in the development 

 of the peripheral nervous system that are not 

 yet clearly understood. The dorsal roots of 

 the spinal nerves arise very early as outgrowths 

 from the sides of the neural plates before the 

 latter fuse together to form the neural canal. 

 They grow down between the neural canal 

 and the myotomes (Fig. 15, ND) and become 

 slightly enlarged, a short distance from their 

 points of origin, to form the spinal ganglia. 

 The ventral roots of the spinal nerves arise 

 later as independent outgrowths from the ven- 

 tral side of the neural canal, and fuse with the 

 dorsal roots distal to the spinal ganglia. 



Development of the Sense Organs 



Since the organs of special sense are chiefly 

 derived from the ectoblast, and since they 

 are all closely connected with the brain, it 

 is well to speak of their development at this 

 point. 



