. The development of the chick; an introduction to embryology . ion, and of the anterior divertic-ulum in its posterior portion. The floor of the intestine be-tween the anterior and posteriorliver diverticula is depressed; laterit becomes separated from theintestinal cavity to form a tem-porary common bile-duct; whichthen receives the two primary di-verticula (Figs. 103 B, 104 and187). The pancreas arises from a dor-sal and a pair of ventral former is an outgrowth ofthe dorsal wall of the intestineimmediately above the posteriorliver diverticulum (Figs. 103 Band 104). At the 35 s
. The development of the chick; an introduction to embryology . ion, and of the anterior divertic-ulum in its posterior portion. The floor of the intestine be-tween the anterior and posteriorliver diverticula is depressed; laterit becomes separated from theintestinal cavity to form a tem-porary common bile-duct; whichthen receives the two primary di-verticula (Figs. 103 B, 104 and187). The pancreas arises from a dor-sal and a pair of ventral former is an outgrowth ofthe dorsal wall of the intestineimmediately above the posteriorliver diverticulum (Figs. 103 Band 104). At the 35 s stage it isa solid thickening of the dorsalwall of the intestine of consider-able extent; a little later the baseof the thickening is hollowed out,and the free margin sends off solidbuds into the dorsal mesenteryjust behind the stomach. The ventral primordia arise from theposterior liver diverticulum in a manner to be described later(Chap. X). Mid-gut. At the 35 s stage the mid-gut is still open to theyolk-sac. Its sul)sequent history is gi\-en in Chapter Fig. 104. — Reconstruction of the liver of the chick at the end of the fourth day of incubation. (After Hammar.) du., Duodenum. L., Substance of liver. Other abbreviations as before. 182 THE 1)I:VI]L0PMENT OF THl] CHICK Anal Plate, Hind-gut, Post-anal Gut, and Allantois. At :il)<)utthe 14 s stage a thickening of the ectotlerm in the mi(hlle hnejust behind the primitive streak extends towards the entodermwhich is folded up so as to nearly meet it, thus cutting off theextra-embryonic mesoblast from the primitive streak. The ecto-derm and the entoderm then come into contact here, and forma firm union, the anal plate (Hg. 70), which is subsequentlyperforated to form the anus. At first, however, the anal plate liesentirely behind the embryo, and the post-anal portion of theembryo arises from the thickened remnant of the primitive streak(tail-bud) whicii grows backwards over the blastoderm beyondthe anal plate. Even before
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