134 THE FROG. 



also in part from the anterior end of the neural ridge. The 

 nerve develops early, and is recognisable before the hatching 

 of the tadpole as a short thick trunk, connecting the side 

 of the brain with the thickened epithelium of the olfactory 

 pit. 



The nerve remains short, up to the time of opening of the 

 mouth, or rather up to the time when the cerebral hemispheres 

 begin to grow forwards. This anterior growth of the cerebral 

 hemispheres is accompanied, as already noticed, by still more 

 rapid growth of the anterior part of the head, in consequence of 

 which the olfactory pits are carried forwards from their original 

 position at the sides of the brain, and become situated in front 

 of it. This causes lengthening of the olfactory nerves, and a 

 change in their direction ; in place of running outwards from 

 the brain, they now run almost directly forwards : the roots of 

 the nerves, however, still arise from the ventral surface of the 

 brain, some distance from its anterior end, as in the adult. 



The sympathetic nervous system. The sympathetic system 

 develops as a series of outgrowths from certain of the cranial, and 

 from all the spinal nerves. These develop ganglionic swellings, 

 which, in the body region (Figs. 84 and 87, NY), lie beneath the 

 notochord, and alongside the dorsal aorta. At an early stage, 

 shortly after the formation of the mouth, the several ganglia of 

 each side become connected together by a longitudinal nerve- 

 cord, but whether this cord arises independently of the ganglia, 

 or, as is more probable, by the formation of outgrowths from the 

 ganglia, has not been definitely determined. 



DEVELOPMENT OF THE SENSE ORGANS. 



The organs of special sensation, like the nervous system 

 itself, are developed from the deeper or nervous layer of the 

 epiblast, and are continuous with their respective nerves from a 

 very early stage in their formation. The derivation of the 

 sense organs from the epiblast is explained by the fact that 

 they are concerned in the appreciation of the presence and 

 nature of external objects, and are therefore necessarily formed 

 on the surface of the body. They are in all cases to be regarded 

 as specially modified parts of the epidermis. 



