280 OUTLINES OF CHORDATE DEVELOPMENT 



an.t. 



Am cay. Ect. 



-Spl'pl. 



FIG. 109. Sagittal sections through the posterior ends of chick embryos. 

 From Lillie (Development of the Chick). A. Median section through an embryo 

 with about twenty-one pairs of somites (about forty-four hours). B. Lateral 

 section through embryo with about thirty-five pairs of somites (about seventy- 

 two hours). All., allantois; Am., amiiion; Am. cav., amnionic cavity; An. pi., 

 anal plate; An. t., anal tube; Ao., dorsal aorta; Bl. v., blood-vessels in wall of 

 allantois; c.C., central canal of spinal cord; Cl., cloaca; Ect., ectoderm; Edam., 

 ectoderm of amnion; E.E.B.C., exocoelom; Ent. endoderm; JV/es., mesoderm; 

 Mesam., mesoderm of amnion; N'ch., notochord; n. T., nerve cord; p'a.G., post- 

 anal gut; p.i.p., posterior intestinal portal; s.A., segmental arteries, between the 

 somites; So'pl., somatopleure; Spl'pl., Sp'pl., splanchnopleure; T.B., tail bud; 

 t.f.So'pl., tail-fold in somatopleure; t.f. Sp'pl. , tail-fold in splanchnopleure. 



