Development of the Third Day 173 



the period of incubation it is as small, relatively, 

 as in the frog ; again : the olfactory lobes in 

 the frog are at first separate, but later fuse to- 

 gether, while in the chick they remain distinct 

 throughout life. There are differences, of 

 course, in the relative sizes of the various parts 

 of the two brains, but, as has been said, the 

 main features in development are essentially 

 the same in the two forms, and a more detailed 

 description, if desired, may be found in larger 

 works. 



The peripheral nervous system. In the de- 

 velopment of the cranial and spinal nerves 

 there is such close resemblance between the 

 frog (page 40) and the chick that but little 

 need be said at this place. They arise (the 

 cranial nerves, perhaps, a little earlier than 

 the spinal) during the latter part of the first 

 or the early part of the second day (Fig. 53), 

 before the medullary folds have fused together 

 along the mid-dorsal line. As in the frog, there 

 are many points in the development of the 

 peripheral nervous system that are still under 

 discussion. 



The sympathetic nervous system. The origin 

 of the sympathetic nervous system in the chick 

 has been the subject of much debate. It is 



