NINE FIGURES
Immediately posterior to the tail thickening of 2-day chick embryos (about 20 somites) there is a structure which is usually interpreted as the remains of the primitive streak (reste de la ligne primitive of Duval, 1889). I n vitally stained embryos this structure is quite apparent. Sagittal sections of such embryos (fig. 2) reveal that this area of the blastoderm is in contact with the apex of a posteriorly directed endodermal diverticulum. The exact appellation of the diverticulum is the subject of some disagreement. (Allantoisbucht of Gasser, 1874, primarer Hinterdarm of His, 1868, anal tube of Lillie, '08, and anal diverticulum of Boyden, '33). Many of the older embryologists (Schauinsland, '02, Balfour, 1881, Duval, 1889, Hertwig, '15 etc.) accepted the iiiterpretation that this gut pocket represented the primordium of the allantois. There does not seem to be any disagreement with the fact that the apex of this pouch is in contact with the anal plate or cloaca1 membrane (Lillie, '08).
Slightly later in development another gut diverticulum is formed, at first anterior and then (as a consequence of the bending of the tail bud) dorsal to the first. Again there is disagreement as to the terminology for this structure (eigentlicher Hinterdam of His, 1868, Enddarm of Gasser, 1874, pars caudalis intestini of Koelliker, 1879, post anal intestine of Lillie, '08, Schwanzdarm of Hertwig, '15). For reasons elaborated later we shall call this pocket the caudal intestine.