280 OUTLINES OF CHORDATE DEVELOPMENT 



sJl. 



Amcav, Ect. 



T.B. 



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). AIL, allantois; Am., amnion; 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; Ectam., 

 ectoderm of amnion; E.E.B.C., exoccelom; Ent. endoderm; Mes., 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. 



