322 IVAN E. WALLIN 



The cephalic extremity of the pharynx projects beyond the oral 

 plate and nearly reaches the floor of the forebrain, a small 

 amount of mesoderm intervening. 



The median thyreoid anlage is a very prominent evagination 

 of the entoderm of the floor of the pharynx. It projects between 

 the layers of splanchnic mesoderm at the arterial end of the 

 heart immediately caudad to the endothelial aorta and the first 

 aortic arches. The cephalic wall of the evagination is consider- 

 ably thicker than the caudal. Cephalad to the thyreoid anlage 

 the first branchial pouches are evaginated from the lateral wall 

 of the pharynx and immediately caudad to the thyreoid the 

 second pair of pouches are present. The first pair of pouches 

 are the larger. Their long axes are directed laterally, cephalad. 

 and slightly dorsal. Opposite the venous opening of the heart 

 the liver anlage is present as a thickening of the gut entoderm. 

 Lung buds have not developed in this stage. 



A cross section of the foregut has a crescentic outline with the 

 concavity directed dorsally. The tube is widest at the point 

 where the first pair of branchial pouches is developed. The 

 cephalic part of the foregut is flattened dorso-ventrally. Cau- 

 dally the dorso-ventral diameter increases gradually to the 

 end of the foregut where it becomes greater than the lateral 

 diamter. 



The gut entoderm extending out into the yolk stalk retains 

 its thickness only a short distance (fig. 3). 



The hind-gut is shorter than the foregut. Its dorso-ventral 

 diameter is comparatively large while its lateral diameter is small. 

 The allantois is evaginated from the ventral wall. The lumen 

 of the diverticulum is very small at its proximal end, but through- 

 out the rest of its extent it is distinct. At first the allantois 

 lies between the aUantoic arteries. At its distal end it comes 

 to lie between the venous and arterial trunks or sinuses of the 

 belly stalk. The end of the allantois is not recurved as found 

 by Lewis ('12) but ends as a straight tube. The hind-gut ex- 

 hibits a dorso-ventral constriction immediately cephalad to the 

 allantoic diverticulum. Caudal to the allantois the hind-gut wid- 

 ens out to form the cloaca. The entoderm of the ventral wall 



