256 CHARLES R. STOCKARD 



THE ORIGIN AND HISTOGENESIS OF VASCULAR ENDOTHELIUM 



AND BLOOD CORPUSCLES AS DETERMINED BY STUDY 



OF MICROSCOPIC SECTIONS 



1. The structure of the heart in embryos without a circulation 



The hearts of the embryos in which there is no blood flow 

 have been described in the hving in the preceding consideration, 

 but when they are studied in section an additional number of 

 very instructive points are brought out. 



In the first place, the heart wall is usually very thin and not 

 well developed. This is particularly true, in the long string- 

 hke hearts that are present in those individuals in which the 

 pericardium is so greatly distended. In the group of figures 

 25 to 28, one sees sections of these hearts taken through various 

 regions. 



Figures 27 and 28 show sections through a long narrow heart. 

 In figure 28 the myocardium is seen to be practically one layer 

 of cells, and within this the endothelial lining is distinctly formed. 

 No noticeable structural difference beyond shght variations in 

 shape can be determined between the nuclei of the mycocardium 

 and those in the endocardium. The myocardial layer is a thick 

 more or less structureless cell mass while the endothelium is 

 w^ell differentiated into a thin single cell layer lining. This con- 

 dition is found in a non-circulating embryo of four days old. 

 Tracing the series towards the conus end of the heart, we find 

 the arrangement indicated in figure 27. The myocardium is 

 here also a thick layer of cells enclosing a distinct endotheUal 

 tube. 



Fig. 25 Section through the heart of a four-day embryo without a circulation; 

 Experiment 11, 1912, Embryo 6. Heart wall poorly formed; large chromatophore, 

 Ch, in wall; -ph., pharynx. 



Fig. 26 Section of a similar heart; Experiment 11, 1912, Embryo 2. The 

 guide outline gives the general relationships of the heart. MC, myocardium; 

 EC, endocardium; Br, brain; p6, periblast nuclei, and, phs, .periblastic material 

 filling the heart cavity, c; red staining cell. 



Figs. 27 and 28 Through the aortic end and figure 28 through the tube-like 

 body of a similar heart; Experiment 11, 1912, Embryo 7. Br, brain; ph, pharynx; 

 £■0, definitely formed endocardium, endothelium; MC, myocardium. The nuclei 

 of the endocardium and myocardium are indistinguishable except for slight dif- 

 ferences in shape. 



