CHANGES DUKING FOUKTH DAY. 



419 



the duodenum, and the pancreas as one solid outgrowtli slightly 

 behind the liver. From the somites now arise the muscle- 

 plates, and the "Wolffian bodies or primitive kidneys become 

 fully developed. 



Changes durinf/ the fourth ihiij. — A still further diminution of 

 white is seen ; the embryo, which has very much grown, lies 

 close to the shell-membrane. More than half the yolk is now 



FiQ. 201. — Head of Embeyo Chick of the Fourth Day. 



CH, Cerebral hemispheres ; FB, vesicle of the third ventricle ; OP, eyeball ; nf, naso- 

 frontal process ; M, cavity of mouth ; SM, superior maxillary process of F. 1, the first 

 visceral fold ; F. 2 and 3, second and third visceral folds ; N, nasal pits ; e, neck be- 

 tween third and fourth visceral folds cut across ; al, alimentary canal ; V, jugular 

 veins ; AO, dorsal aorta ; uc, neural canal ; cli, notochord. (After Foster and Balfour.) 



invested by the germinal membrane, and the blood-area is about 

 an inch in diameter. The amnion obscures the view of the 

 chick, which lies beneath it. Both cranial flexure and tail-fold 

 have much increased, and thus the embryo appears spirally coiled. 

 On each side of the chick is now seen a long ridge — the Wolffian 

 ridge ; on this there appear two anterior and two posterior bud- 

 like outgrowths, tlie liinbs. The anterior buds or wings arise near 



