CHICK, 68 TO 72 HOURS' INCUBATION 31 



is its shape in the section? Is the body closed ventrally? 

 What is the shape of the body cavity? Locate the Wolf- 

 fian ducts. Are the Wolffian bodies in this section? 

 The thickening of the body wall to the sides of the body 

 cavity are the hind-limb buds. Does the amniori entirely 

 enclose the body? 



Draw the section, color the different germ layers, and 

 label all the parts. 



c. Reconstruction 



(i) The Alimentary Tract and Its Appendages 



Beginning with the most anterior part, trace the ali- 

 mentary tract through all the sections. It will be seen 

 that from the most anterior part to the region of the 

 vitelline veins it is an irregular tube. This part of it is 

 the fore-gut. From the fore-gut posteriorly for about a 

 third or a fourth of the length of the embryo the alimen- 

 tary tract is continuous with the yolk sac. In the poste- 

 rior part it again forms a tube, the hind-gut. The 

 alimentary tract continues to fold off from the yolk sac 

 until the fore-gut and the hind-gut meet. They meet in 

 - the posterior part of the duodenum. The most anterior 

 part of the fore-gut, that is, the portion beneath the 

 fore-brain, is comparatively small. This is the hypoph- 

 ysis. Suddenly it becomes very large and irregular. 

 The lateral irregularities are the visceral pouches and 

 clefts. How many? Ventral to the second visceral 

 pouch is a long, narrow evagination, the thyroid gland. 

 The pharynx becomes flattened laterally to form the 

 trachea, and then the lung-buds are given off. Poste- 



