EMBRYO WITH EIGHT SEGMENTS. 



189 



can recognize the future muscular heart, as it is sometimes called. The few cells 

 above described (Fig. 138, Endo) have increased considerably in number and have 

 joined themselves together in such a manner as to indicate clearly the formation of 



FIG. 138. SECTION OF A CHICK EMBRYO WITH SEVEN SEGMENTS. TRANSVERSE SERIES 510, SECTION 184. 



Am.ves, Amnio-cardiac vesicle. EC, Ectoderm. Endo, Cells forming the anlage of the endothelialjheart. Md, 



Hind-brain, mes, Mesenchyma. msth, Anlage of mesothelial heart. Ph, Fore-gut. 



m.ht 



pro. am 



FIG. 139. CHICK EMBRYO, TRANSVERSE SECTION ACROSS THE ANLAGE OF THE HEART IN A STAGE SLIGHTLY MORE 



ADVANCED THAN FIGURE 138. 



Md, Wall of medullary tube, nch, Notochord. msth, Mesothelium. Ph, Pharynx, pro.am., Tip of pro-amnion. 



en.ht, Endothelial heart, m.ht, Muscular heart. 



the endothelial heart (Fig. 139, en.ht). At first the cells are irregularly disposed and 

 have several irregular cavities between them, which soon fuse so as to form two 

 main cavities running longitudinally. As the two cavities enlarge they meet in the 

 median line and remain separated at first by a wall of two layers of endothelium. 



