EMBRYO WITH EIGHT SEGMENTS. 



177 



the cephalic end of the embryo (Fig. 129). It should be noted that this figure was 

 drawn from a hardened, and not from a fresh specimen. The head of the embryo 

 lies toward the large end of the area pellucida and projects freely above the sur- 

 face of the germinal area. Underneath the projecting head is a very clear area 

 with distinct lateral boundaries. It is called the pro-amnion and contains no meso- 

 derm whatever. Near the head are two characteristic areas, one on each side, easily 

 recognized by the fact that the surface of the germinal area rises like a dome over 



FIG. 129. CHICK EMBRYO AFTER TWENTY-SEVEN HOURS' INCUBATION, WITH EIGHT PRIMITIVE SEGMENTS. 



ALCOHOLIC SPECIMEN, x 15 diams. 



each space. The spaces are termed the amnio-cardiac vesicles. They are in reality 

 local expansions of the ccelom which cause the somatopleure, or upper leaf of the 

 germinal area, to arch upward on either side of the embryo. By the study of cross- 

 sections (Fig. 136) the relations may be clearly understood. The posterior limit of 

 the head is marked by a curving line (to see this sharply the focus must be 

 lowered) the cavity of which faces the caudal end of the embryo. This line 

 marks the position of the fovea cardiaca and from it the fore-gut extends into the 

 head of the embryo. On the cephalic side of the fovea and underneath the fore- 

 gut the heart will be developed. On the sides of the fovea, running forward 



