'EMBRYO WITH EIGHT SEGMENTS. 



187 



served: The ' epidermis, EC, of the embryo is thickened and fits closely against the 

 dorsal portion of the hind-brain, with which it is actually fused in the median line. 

 The ganglionic crest is represented only by a few more lightly stained cells at the 

 junction of the epidermis with the medullary wall, M d, but is much more developed 

 in nearby 'Sections, both cephalad and caudad. The fore-gut, Ph, is very wide, the 

 entoderm on its dorsal side is very thin, but grows thicker toward the lateral 

 boundaries of the gut, and is thickest in the mid-ventral line, where it forms a 

 shallow median groove; the nuclei in this groove are all next the external surface 

 of the entoderm. The mesothelium, msth, is a thin layer, which above the 

 amnio-cardiac vesicles enters into the formation of the somatopleure or true body 



msth. mes. Ao.d. 



A.c.v Endo. m.Ht. nch Ph. Spl. 



FIG. 136. SECTION OF A CHICK EMBRYO WITH EIJGHT SEGMENTS. TRANSVERSE SERIES 642, SECTION 114. 

 A.c.v, Amnio-cardiac vesicle. Ao.d, Dorsal aorta, c, Line of Closure of the medullary canal. EC, Ectoderm.. 

 Endo, Endothelial heart. Md, Wall of hind-brain, mes, Mesenchyma. m.Ht, Mesothelial heart, msth, 

 Mesothelium. nch, Notochord. Ph, Fore-gut. Spl, Splanchnopleure. X IO diams. 



wall. It is not sharply separated from the mesenchyma, mes, as can be very well 

 seen in the part of the layer underneath the pharynx. When the heart is reached 

 the mesoderm forms a wide duplicature, m.Ht, } the mesothelial heart, which is a 

 layer of much greater thickness than the mesothelium proper, and which offers the 

 important characteristic that it shows no differentiation into mesothelium and 

 mesenchyma. -Between the mesothelial heart-tube and the endothelial, Endo, there 

 is a wide space which contains no visible structures, hence we assume that the 

 two cardiac tubes are kept apart by fluid only. Beneath the ccelom (amnio-cardiac 

 vesicles, A.c.v) is the Splanchnopleure, Spl, which has two thin layers: the upper 

 is mesoderm, the lower entoderm. The mesoderm has numerous nuclei, and if 

 followed out laterally to the area opaca will be found affixed to blood-vessels and 

 blood-islands, which together constitute the angioblast or anlage of the vascular sys- 

 tem. It can be observed in most places readily that the angioblast lies beneath the 

 mesoderm proper and is distinct from it. The entoderm has few nuclei and in 

 the area pellucida is very thin, but where it passes to the area opaca it gradually 

 but rapidly thickens, and .is composed of very large columnar cells (compare Fig. 30) 



