FOWL— SECOND DAY 



531 



clearly an early stage in the folding off of the foregut from the 

 cavity of the yolk-sac — a fold of splanchnopleure growing inwards on 

 each side below what will become the foregut. The large vessels 

 seen in the splanchnopleure, external to the fold just mentioned are 

 tributaries of the vitelline veins, and a few sections farther forwards 

 they would be found to be united together to form the main vitelline 

 vein on each side. 



As the series of sections is traced forwards the two folds 

 of the splanchnopleure are seen to approach one another and 

 finally to meet and undergo fusion, so that there now exists a 

 foregut cavity shut off (as seen in transverse section) from the 

 yolk-sac, the walls of the two structures being still connected by a 

 median vertical partition formed by the fusion of the endoderm from 



ixmc. 



enc. 



Pis. 234c- 



- Transverse section of a second-day Fowl embryo passing through 

 the rudiment of the heart. 



A, dorsal aorta ; d.mc, dorsal mesocardium ; enc, endocardium ; end, endoderm ; f.g, foregut ; me, 

 myocardium ; s.c, spinal cord ; so.m, somatic mesoderm ; sp.m, splanchnic mesoderm ; splc, splancli- 

 nocoele ; v.me, ventral mesocardium. 



the two sides. A little farther forward this partition disappears 

 from the section and the foregut as seen in section (Fig. 234c) is 

 quite isolated from the endoderm of the yolk-sac wall. The vitelline 

 veins have also fused to form the tubular heart. It is seen that the 

 splanchnic mesoderm ensheaths the endothelial wall of the heart 

 (enc) on each side and that where it does so it is somewhat thickened 

 (mc) as compared with the same* layer in the region overlying the 

 yolk-sac. This localized thickening of the splanchnic mesoderm is 

 destined to give rise to the entire thickness of the heart wall except 

 the lining endothelium. It is seen to be continuous with the extra- 

 cardiac portions of the splanchnic mesoderm by the dorsal (d.mc) 

 and ventral mesocardium (v.mc). 



Traced forwards through the series of sections the heart is seen 

 to narrow in calibre as it tapers off into the ventral aorta. Towards 

 its front end the latter gives off a large branch on each side which 



