STRUCTURE OF THIRTY-THREE HOUR CHICKS 69 



somewhat detached from the mesial face of the mesoderm layer. 

 These cells soon become organized to form the endocardial 

 primordia. 



In a chick of about 26 hours, sections through a corresponding 

 region show distinct dfferentiation of the endocardial and epi- 

 myocardial primordia (Fig. 26, B). The endocardial primordia 

 are a pair of delicate tubular structures, a single cell in thick- 

 ness, lying between the entoderm and mesoderm. They arise 

 from the cells seen separating from the adjacent thickened meso- 

 derm in the 25-hour chick. As their name indicates they are 

 destined to give rise to the endothelial lining of the heart. By 

 far the greater part of each of the original mesodermic thicken- 

 ings becomes applied to the lateral aspects of the endocardial 

 tubes as the epi-myocardial primordium which is destined to 

 give rise to the external coat of the heart (epicardium) and to 

 the heavy muscular layers of the heart (myocardium). 



In chicks of 27 hours the lateral margins of the anterior intes- 

 tinal portal have been undergoing concrescence lengthening 

 the fore-gut caudally and involving the heart region. In this 

 process the former lateral margins of the portal swing in to 

 meet each other and fuse in the mid-line, and the endocardial 

 tubes of the right and left side are brought toward each other 

 beneath the newly completed floor of the fore-gut (Figs. 26, C 

 and 27, B). In the 28-hour chick the endocardial primordia 

 are approximated to each other (Figs. 26, D and 27, C) and by 29 

 hours they fuse in their mid-region to form a single tube (Figs. 

 26, E and 27, D). 



At the same time the epi-myocardial areas of the mesoderm 

 are brought together first ventrally (Fig. 26, Z>) and then dor- 

 sally to the endocardium (Fig. 26, E). Where the splanchnic 

 mesoderm of the opposite sides of the body comes together dor- 

 sal and ventral to the heart it forms double layered supporting 

 membranes called respectively the dorsal mesocardium and the 

 ventral mesocardium. - j The ventral mesocardium is a transitory 

 structure, disappearing almost as soon as it is formed (Fig. 26, 

 E). The dorsal mesocardium, although the greater part of it 

 disappears in the next few hours of incubation, persists in em- 

 bryos of the stage under consideration, suspending the heart 

 in the pericardial region of the ccelom. Conditions reached in 

 the heart region at 33 hours of incubation are shown in section 



