THE VASCULAR SYSTEM 



369 



diameter as the erythrocytes. Their source is both in the bone-marrow 

 and the lymph glands. The granular leucocytes arise in the bone marrow 

 as myelocytes. They become differentiated mostly into neutrophiles, 

 the nuclei of which have diverse shapes while the cytoplasm is neutral 

 in its staining reactions. A small proportion of the myelocytes become 

 eosinophiles, which have a coarsely granular cytoplasm with a strong 

 affinity for eosin and other acid dyes. The cytoplasm of other leucocytes 

 stains with basic dyes. They are, consequently, known as basophiles. 



ural plate 



C D 



Fig. 307. — Sections cut transversely through the cardiac region of pig embryos of 

 various ages to show the origin of the heart from paired primordia. A, 5-somite embryo; 

 B, 7-somite embryo; C, lo-somite embryo; D, 13 somite embryo. (Projection diagrams 

 X50, from series in the Carnegie Collection.) (From Patten's "Embryology of the 

 Pig.") 



Development of the Heart. The heart makes its appearance as a 

 two-layered tube ventral to the pharynx, so that the early embryo has its 

 heart in its throat. Of the two layers, the inner becomes the endothelial 

 lining of the heart, while the outer forms the epicardium and the muscular 

 myocardium. The right and left halves of the heart begin as longitudinal 

 folds of the splanchnic mesoderm. Between these mesodermal folds 

 and the adjacent endoderm, scattered mesenchyme cells appear, and soon 

 become arranged as a thin-walled endothelial tube in each of the folds. 



The paired mesodermal folds with their enclosed endothelial tubes 

 arise before the ventral wall of the pharynx is formed. The union of the 

 two halves occurs in correlation with the formation of the floor of the 



