CHICK, 33 TO 38 HOURS' INCUBATION 19 



the mesoderm which lies next to it forms the somato- 

 pleure, and the endoderm with the mesoderm lying next 

 to it forms the splanchnopleure. The space or cavity 

 between the splanchnopleure and the somatopleure is the 

 body cavity. If the body cavity is in the embryo, it is 

 called the embryonic body cavity or ccdome. If it is in the 

 blastoderm and not in the embryo, it is called the extra- 

 embryonic body cavity. Is there a ccelome in the section 

 in the region of the fore-brain? The ectoderm forms 

 the outer covering of the head. Is it of uniform thick- 

 ness? If not, where is it thickest? The fore-brain is 

 ectodermal in origin. The optic vesicles are evaginations 

 from either side of the fore-brain and are continuous with 

 it. Is the cavity of the fore-brain continuous with the 

 cavities of the optic vesicles? Is there any mesoderm in 

 this section? If so, where? What part is endoderm? 

 It will be observed that the extra-embryonic blood 

 vessels are, for the most part, in the mesoderm of the 

 splanchnopleure . 



Draw the section and label all the parts. Color the 

 germ layers. 



(2) Through the Middle of the Heart 



What is the shape of the heart in cross section ? Com- 

 parative size? The heart is mesodermal in origin. The 

 inner, thin layer of cells is the endocardium and forms the 

 endothelial lining of the heart. The outer, thicker layer 

 is the myocardium and forms the muscle of the heart. In 

 the splanchnic layer of mesoderm are numerous spaces. 



