200 EMBltYOLOGY. 



and the edge of the cup-like depression. The cornea is of later 

 formation, and is derived from the epiblast of the skin. 



How is the auditory apparatus developed ? 



Very early in the life of an embryo there is a depression on 

 either side of the head which passes through the same process as 

 that mentioned for the crystalline lens and for the germinal epi- 

 thelium in the formation of ova. The mass of epiblast thus sepa- 

 rated forms the epithelium of the labyrinth and vestibule, the 

 surrounding mesoblast furnishing the bony and muscular structures. 

 The auditory nerve is developed with other cranial nerves, and 

 grows in to its end-organs from its central origin. 



How is the olfactory apparatus derived ? 



In a similar way to the internal ear and the lens. The nasal fossa 

 is primarily a depression in the superficial epiblast, which widens 

 and deepens arid receives the nerve-filaments from the olfactory 

 lobe. This lobe is originally a bud from the prosencephalon. The 

 primary olfactory depression continues to widen until it opens into 

 the mouth, and is again shut off by the growth of the branchial 

 arch, which forms the superior maxilla. The nose is similarly 

 derived from the mesial and lateral nasal processes. 



Describe the method of the development of the alimentary canal. 

 As has already been explained, the primitive alimentary canal is 

 formed from the involution of the splanchnopleure, and is really a 

 portion of the yelk-sac partially shut off from the rest. It is at 

 each end a blind pouch which follows the head and tail folds. 

 The portions have received the names fore-gut and hind-gut as they 

 occupy one or other of these folds. 



How does the fore-gut become changed? 



It joins with the mouth -cavity by the folding back of the epi- 

 blast in the formation of the branchial arches, and from it are 

 formed the pharynx, oesophagus, and stomach. 



How does the hind-gut approach the surface ? 



By a similar involution of the epiblast the anus and about half 

 of the rectum are formed, into which the hind-gut opens to 

 complete the alimentary tract. 



What deformities in the new-born depend upon the defective 



accomplishment of these changes ? 

 The oesophagus is sometimes impervious at birth, and the rectum 



