EMBRYO WITH TWENTY-EIGHT SEGMENTS. 



281 



plate, and on the ventral side, where they form the floor-plate; the cavity is nar- 

 row and slit-like. The notochord lies close under the ventral side of the 

 medullary tube and below it is the median dorsal aorta, Ao, a single and very 

 large vessel, which is formed by the union of the two dorsal aortae shown in figure 

 1 57, Ao. D. Immediately below the aorta follows the pharynx, Ph, which is now 



Raph. 



FIG. 158. SECTION OF CHICK EMBRYO WITH ABOUT TWENTY-EIGHT SEGMENTS. TRANSVERSE SERIES 92, 



SECTION 144. 



Am, Am', Amnion. Ao, Aorta. Au, Cardiac auricle. Cho, Chorion. Cos, Ccelorh. D. C, Duct of Cuvier. 

 EC, Ectoderm. Endo, Endothelial heart, m.ht, Muscular heart, msth, Mesothelium. My, Primitive 

 segment. Ph, Pharynx. Raph, Raphe of Amnion. Soin, Somatopleure. Sp.c, Spinal cord. Ven, Ventricle 

 of heart. X 5 diams. 



more rounded in form and does not extend far laterally. Its entodermal lining 

 is moderately thick, but it is somewhat thinner near the median dorsal line. On 

 either side of the pharynx the mesothelial layer is very thick and stands out con- 

 spicuously, owing to its dark staining. Above the pharynx it thins out and 



