186 



STUDY OF YOUNG CHICK EMBRYOS. 



Below the ccelom is another cavity, Pro.am, that of the pro-amnion, lined by ecto- 

 derm and opening anteriorly. This structure is not further dealt with here, partly 

 because its history is complicated, partly because it does not occur in the human 

 embryo. The ventral aortae (Fig. 134, Ao.v) have united into a single blood-chan- 

 nel, Ht, which we can identify as the blood-channel of the heart; it is called the 

 endothelial heart. The mesothelium, msth, on the dorsal side of the ccelom forms 

 a protuberant fold, the mesothelial heart, which surrounds the inner vascular space. 

 The two heart-walls are some distance apart. The inner heart produces only the 

 lining endothelium of the adult organ, the mesothelial heart produces all its muscu- 

 lar and connective-tissue components, and also the pericardium. The difference 



mes. Ao.d. 



Aid. 



Coe. Ent. 



nth. Pro.am. msth. 



FIG. 135. SECTION OF CHICK WITH EIGHT SEGMENTS. TRANSVERSE SERIES 86, -SECTION 86. 

 Ao.d, Dorsal aorta. Coe, Ccelom. EC, Ectoderm. Ent, Entoderm. G. Ganglionic crest. Ht, Aortic end of 

 the endothelial heart, nch, Notochord. Md, Hind-brain, mes, Mesenchyma. msth, Mesothelium. Ph, 

 Fore-gut. Pro.am, Pro-amnion. X 100 diams. 



in the form of the cross-sections of the hind-brain, Md, mid-brain, and fore-brain 

 should be noted. Between the brain and fore-gut, and touching both, lies the small 

 notochord, nch, with a sharp outline. The ganglionic crest, G, of the mid-brain is 

 still traceable, but occupies a much smaller area, than in figure 134. In the mid- 

 dorsal line the crest, G, the epidermis, EC, and the medullary tube, Md, are fused 

 together. In correspondence with the reduction of the crest, the area occupied by 

 the true mesenchyma, mes, is increased. 



Section through the Cephalic (Aortic) End of the Heart (Fig. 136). The general 

 topography is similar to that of figure 135. The most striking differences are, that 

 in the present section the heart is very much larger, is bent to the right of the 

 embryo (the left of the figure), and has a narrow connection (mesocardium) with 

 the floor of the pharynx; that the ccelom is much expanded to form the amnio- 

 cardiac vesicles, A.c.v, one on each side, which are continuous with one another 

 O n the ventral side of the heart; that the lips of the medullary tube are in contact 

 a t c, but have not actually fused; and that there is no pro-amnion, because it does 

 n ot extend so far back under the embryo. The following details should be ob- 



