EMBRYO WITH SEVEN SEGMENTS. 



301 



are lined by a distinct layer of mesothelium, msth, which, for the most part, 

 is very thin, but underneath the wide pharynx the mesothelium has acquired 

 considerable thickness and is somewhat irregular, as shown in the figure. Be- 

 tween the mesothelium and the floor of the fore-gut. Ph. there lie a few cells, 

 Endo, which are the precursors of the endothelium of the heart; the thickened 

 mesothelium forms the muscular wall of the heart. In the figure there is a 

 median partition of mesodermic cells by which the two amnio-cardiac vesicles 

 are separated from one another. During this stage, or very soon after, this 

 partition breaks down and disappears, and thereafter the two vesicles com- 

 municate freely across the median line, and the pericardial chamber is said 



FIG. 172. CHICKEN EMBRYO, TRANSVERSE SECTION ACROSS THE ANLAGE OF THE HEART IN A STAGE 



SLIGHTLY MORE ADVANCED THAN FlG. 1 71. 



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

 amnion. En.ht, Endothelial heart, m.ht, Muscular heart. 



to be formed, though it is not yet delimited from the general coelom of the 

 vesicles. 



The further development of the heart may be understood by the examina- 

 tion of a somewhat older stage (Fig. 172). As shown in the illustration, the 

 mesothelium has become very protuberant, m. hi, in the median line under- 

 neath the fore-gut, Ph. On either side it rapidly thins out, msth. In the 

 protuberant fold we can recognize the future muscular heart, as it is sometimes 

 called. The few cells (Fig. 171, Endo} above described have increased consider- 

 ably in number and have joined themselves together in such a manner as to 



