2O LABORATORY OUTLINES FOR EMBRYOLOGY 



These are cross sections of extra-embryonic blood vessels. 

 Dorsal to the heart is a rather large cavity, bounded 

 ventrally by a thick wall and dorsally by a thin one. 

 This is a portion of the fore-gut, the pharynx. The 

 pharynx is lined with endoderm. In the median line of 

 the body, dorsal to the pharynx, is a circular mass of 

 cells, the notochord. The notochord is endoderm. On 

 each side of the notochord and dorsal to the pharynx 

 is a rather large cavity. These are dorsal aortce. Why 

 is there no ventral aorta? All blood vessels are formed 

 in mesoderm. What is the neural tube called in this 

 region? The ectoderm of the outside of the body is con- 

 tinuous with the ectoderm of the blastoderm. Is the 

 ccelome separated from the extraembryonic body cavity? 

 Draw the section, label all the parts, and color the germ 

 layers. 



(3) Through the Region of the Mesoblastic Somites 



The mesoblastic somites are on either side of the neural 

 tube. They are blocks of mesodermal cells which radiate 

 from a common central point. Connecting each meso- 

 blastic somite with the mesoderm of the somatopleure 

 and the splanchnopleure is a neck of cells. This neck of 

 cells is the intermediate cell mass or nephrotome. The 

 mesodermic layers of the splanchnopleure and the 

 somatopleure are called the lateral plates. Compare the 

 size of the dorsal aortae here with those in section (i). 



Draw the section, label all the parts, and, color the germ 

 layers. 



