'274 THE CHICK. 



the muscle plates. By the end of the fourth day the nerves have 

 doubled in length, and the primary dorsal and ventral divisions 

 are already established, each division including fibres from both 

 the dorsal and ventral roots. 



In the part of the body between the fore and hind limbs 

 (cf. Fig. 115), the main branches of the nerves run in the body 

 wall or somatopleure. In the segments opposite the limbs, 

 the nerves enter the limbs and divide into dorsal and ventral 

 branches, which unite with the corresponding branches of the 

 nerves in front of, or behind, them to form broad plates of nerve 

 fibres, from which the individual nerves of the adult limb arise. 



d. The Sympathetic Nervous System. The origin of the 

 sympathetic nervous system in the chick has been much debated, 

 and is not yet satisfactorily determined. The most trustworthy 

 observations are to the effect that the sympathetic nervous system 

 arises at an early stage, the third or fourth day, as a series of 

 outgrowths from the spinal nerves, apparently derived directly 

 from the spinal ganglia. These grow inwards, at a level im- 

 mediately above the cardinal veins, and close to the dorsal 

 aorta : at their ends are ganglionic enlargements, the nerve- 

 cells of which are apparently derived, by direct migration, from 

 the spinal ganglia. These ganglionic enlargements soon become 

 connected, along each side of the body, by longitudinal com- 

 missures, apparently formed by outgrowths of nerve-fibres from 

 tfye ganglia themselves. 



If this account is correct, the sympathetic nervous system 

 of the chick is to be regarded merely as a specialised part of 

 the spinal nervous system. 



DEVELOPMENT OF THE SENSE ORGANS. 



The general history of the development of the sense organs 

 in the chick is very similar to that already described in the 

 frog. In all cases the essential part of the organ, the actual 

 sensitive surface itself, is derived directly or indirectly from the 

 epiblast or epidermis. 



1. The Nose. 



The olfactory organs appear, about the fiftieth hour, as a pair 

 of thickened patches of the external epiblast on the under 



