AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



side on the yolk, and the cranial flexure round the notochord 

 is well-marked. Five visceral arches and four visceral clefts 

 have made their appearance (Fig. 80), one less than in the 

 frog. The most posterior of these are very small. The pro- 

 minent heart is situated just under the throat, close to the 



n 



Fig. 79. SUKFACK VIK\V OF SECOND DAY CHICK Magnified rather more 

 than 50 diameters (from KSlliker). Ap, Area pellucida ; Vh, Mh, JHh, 

 fore-, mid-, and hind-brains. The neural groove has closed as far back 

 as omr. Pr, Primitive streak. Eight mesodermic somites are shown. 



ventral ends of the visceral arches. The eye and ear are much 

 more advanced, and the olfactory sacs are commencing as pits. 

 On the fourth day the limbs appear as buds. The chick is 

 hatched on about the twentieth day of incubation, breaking the 

 shell with its beak, which possesses a small knob for the purpose. 



