418 EMBEYOLOGY OF THE CHICK. 



optic vesicles becoming folded in, in front, producing a cup- 

 shaped depression, and the optic stalk becomes small and con- 

 verted into the optic nerve. From this cup-shaped vesicle the 

 various parts of the eye arise. The epiblast over the vesicles 

 becomes thickened and folded into them ; the mouth of the 

 vesicle is closed, and the whole part pushed in breaks off 

 and forms the lens. The mesoblast round the optic vesicles 

 gives rise to the choroid and sclerotic, whilst in front the meso- 

 blast grows over the lens and forms the cornea, the epiblast 

 over it simply forming the epithelial covering. The organs of 

 hearing and smell also arise as epiblastic invaginations, the 

 former showing on the second day, the latter only on the third. 



In the neck region appear those curious structures found in 

 all vertebrates, the visceral clefts. The visceral clefts are fissures 

 that pass through the walls of the throat to the pharynx. 

 There are four on each side, and they are formed by a pushing 

 out of the internal hypoblast and a pushing in of the external 

 epiblast until a perforation is made. On the anterior border of 

 each cleft is a thick fold, the visceral fold (fig. 201, i^ 1 to F Z), 

 the fourth cleft having two folds. These visceral clefts are rem- 

 nants of pre-existing gill-slits. The first pair of folds only remain 

 in a modified form, taking part in the formation of the mouth 

 and mandibles. They each send off a branch during the third 

 day to the anterior edge ; these two branches nearly meet, but 

 are separated by a median process. Between the main folds a 

 space appears ; this becomes the mouth {M). The second and 

 third arches partly remain as the hyoid bones, the last two 

 becoming quite obliterated. The first visceral cleft persists, 

 being connected with the auditory organ, becoming the Eus- 

 tachian tube and tympanic cavity. 



The three parts of the alimentary canal on the third day unite. 

 From the oesophagus the lungs arise as bud-like outgrowths, 

 Liver and pancreas also make their appearance by a similar 

 process, both the latter arising between the fifty -fifth and 

 sixtieth hours of incubation — the liver as two diverticula from 



