20 LABORATORY OUTLINES FOR EMBRYOLOGY 



These are cross sections of extra-embryonic blood vessels. 

 Dorsal to the heart is a rather large hole, bounded ventrally 

 by a thick wall and dorsally by a thin one. This is a por- 

 tion 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 noto- 

 chord and dorsal to the pharynx is a rather large hole. 

 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 continuous with the ectoderm of the 

 blastoderm. Is the coelome separated from the extra- 

 embryonic 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 somat- 

 opleure 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. 



