282 



THE CHICK. 



constriction of the embryo from the yolk-sac proceeds, the fore- 

 gut and hind-gut lengthen at the expense of the mid-gut ; and 

 after about the seventh day, when the yolk-stalk is reduced to a 



BM 



Mf-1 



KT 



FIG. 123. A median longitudinal, or sagittal, section through a Chick Embryo 

 at the end of the fifth day of incubation ; the section is taken strictly in the 

 median plane, except as regards the Wolffian body and kidney, which are 

 introduced in the figure in order that their relations to the alimentary 

 canal may be shown. The optic lobe, cerebral hemisphere, and optic stalk 

 of the left side are shown in perspective. The amnion has been removed, 

 and the allantois and yolk-stalk cut short close to the embryo. (Compare 

 Figs. 100 and 115 for surface views of embryos of the same age.) x 12. 



A, dorsal aorta. BF, third ventricle, or cavity of thalamencephalon. BH, fourth 

 ventricle, or cavity of medulla oblongata. BL, cerebellum. BM, cavity of mid-brain. 

 BS, cavity of the vesicle of the hemispheres. CH, notochord. GrH, hind-gut. G-T, 

 mid-gut. 'IN. infundibulnm. KG, Wolffian duct. KD, ureter. KM, Wolffian body. 

 KT, kidney. LGr. lung. MM", uiandibular arch. NS, cavity of spinal cord. PIST, 

 pineal body. PT. pituitary body. R,T, truncus arteriosus. JETV, ventricle of heart. 

 TA, stalk of allantois, cut short. TC, cloaca. TH, thyroid body. TP, pharynx. 

 TS, stomach. "W. liver. WD, bile duct. YS, yolk-stalk, cut short. 



very narrow tube, and the walls of the mesenteron are complete 

 along its whole length, the mid-gut, as a distinct portion of the 

 alimentary tract, ceases to exist. 



