NORMAL HISTOLOGY. 



earliest blood-vessels and the first blood-cells. The blood-channels 

 appear within the nucleated ' ' islands' ' as spaces which follow the 

 partial breaking down of the inner portions of the areas. The 

 peripheral zone of the nucleated cell-mass becomes the endothelium 

 of the future blood-vessel, while, probably, certain of the enclosed 

 mesodermic elements persist as the primary blood-cells. After a 

 time the mesoderm surrounding the newly-formed endothelial tube 

 differentiates into the muscular and other tissue of the remaining 

 coats. The endothelium is, therefore, genetically the oldest part 

 of the vessel, although its characteristic appearance, as seen in 

 silvered adult tissue, is not visible until further differentiation has 

 taken place. 



The blood-channels are further extended by the fusion of elongated 

 mesoblastic cells with those of the walls of the primary vessels, the 

 lumina of the latter gradually entering the solid processes, which are 

 thus converted into tubes. After the development of the earliest 

 vessels in the manner indicated, the formation of all new vessels 

 subsequently, in pathological processes as well as in normal ones, 

 is associated closely with the connective-tissue cells, since 

 solid protoplasmic processes of the united cells become later the 

 walls of the young vessel. 



The development of the heart resembles that of the extra- 

 embryonic vessels in so far that the part first formed the primary 

 endothelial tube originates by the differentiation of the mesodermic 

 cells and the hollowing out of the tissue 

 lying enclosed. In its very early stage the 

 mammalian heart exists as two distinct and 

 widely-separated tubes, which later unite to 

 form a single sac. Outside the primary 

 endothelial heart the mesoderm differ- 

 entiates into the muscular tissue of the 

 Section of a pan of the develop- cardiac wall, but for some time the endo- 



ing heart of a rabbit embryo of .... . . . 



eleven days: ,, the endothelial thelial and muscular layers continue as 

 tube, within which lie several of independent tubes, the inner endothelial 



the primary nucleated red blood- < 11 



ceils (*>; m, the slightly differ- llnln g appearing as a shrunken cast repro- 

 entiated mesoblastic cells, which ducing the contours of the larger muscular 



later become the muscular tissue. /-r-i i i 



organ. The two tunics are connected by 



numerous bridging bands, which increase in number and size with 

 the progress of the development of the organ ; these primary tra- 

 beculae are represented in the adult organ by the columnae carneae 

 and musculi pectinati. The pericardium originates as the special- 

 ized layer of mesoderm the mesothelium forming the immediate 

 boundary of the general primary body-cavity, of which the peri- 

 cardial sac is only a constricted portion. 



