130 DEVELOPMENT OF THE FROG. 



very early stage, while the neural groove is still shallow and 

 open : they are, therefore, from their first appearance con- 

 tinuous with the brain or spinal cord. 



The ventral roots of the spinal nerves arise later than the 

 dorsal ones, as outgrowths from the cord near its ventral surface. 

 They are at first independent of the dorsal roots, but soon 

 become connected with these. 



G. Development of the Sense Organs. 



The organs of special sensation are developed from the deeper 

 or nervous layer of the epiblast, and become connected with 

 their respective nerves at a very early stage of their formation. 



The derivation of the sense organs from the epiblast is 

 explained by the fact that they are concerned with the appre- 

 ciation of the presence and nature of external objects, and are 

 therefore necessarily formed on the surface of the body. They 

 may be regarded as specially modified portions of the epidermis. 



The Nose. The olfactory organs appear at a very early stage 

 as paired thickenings of the nervous layer of the epiblast at the 

 anterior end of the head, in the angles between the fore-brain 

 and the optic vesicles. A pitting-in of the surface, involving 

 both layers of the epiblast, soon appears in each of these 

 thickenings, and the pits so formed become the nasal sacs ; the 

 mouths of the pits forming the nostrils or anterior nares, and 

 the epiblast lining the pits giving rise to the olfactory epithelium. 



From the inner or deeper end of each olfactory j^it a diverti- 

 culum, at first solid, but soon becoming hollow, grows down- 

 wards to the roof of the pharynx, into which it opens, as the 

 posterior nares, very shortly after the formation of the mouth 

 opening. 



The Eye. The eye differs from the other sense organs, inas- 

 much as the lens alone is formed directly from the surface epi- 

 blast, while the sensitive part of the eye, or retina, arises as an 

 outgrowth from the brain. The optic vesicles have already been 

 described as arising at a very early period as lateral outgrowths 

 from the fore-brain ; these soon become constricted at their 

 necks so as to be connected with the brain by narrow stalks, 

 which ultimately become the optic nerves. 



The outer surface of each optic vesicle, which is at first in close 

 contact with the surface epiblast, soon becomes flattened, and 

 then thickens so greatly as almost to obliterate the cavity of the 



