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MISS K. M. PARKER ON THE 



con.t<act with the entoderm (text-fig. 2). In the region of the 

 maximum size and development of the pleuro-pericardial canals 

 (PI. I. fig. 1) the endothelial primordia of the heart are 

 differentiating (text-fig. 3, End.) between the entoderm and the 

 splanchnic mesoderm, which therefore projects as a prominent 

 fold into the pleuro-pericardial canal. 



Text-figure 3. 





Ferameles ohesula (IZ, B). 



A. Transverse section hi region of greatest widtli of pleuro-pericardial canals, 

 with endothelial tubes developed. B. Transverse section showing origin of 

 angioblast cells from the splanchnic mesoderm. 



A.C. Angioblast cell. Mid. Endothelium. M.P. Medullary plate. F.p.C. Pleuro- 

 pericardial canal. 8.M. Splanchnic mesoderm. So.M. Somatic mesoderm. 



The endothelial heart primordia are best developed in their 

 posterior portions, where they are actually tubular in some 

 embryos of this stage (see text-fig. 3 A). Anterior to the tubular 

 portion, the primordia are represented by solid cords of angio- 

 blast cells, isolated examples of which are found scattered along 

 the length of the pleuro-pericardial canals in the positions indi- 

 cated in PI. I. fig. 1 (A.C). It may be concluded from this that 

 the endothelial tubes differentiate postero-anteriorly, and that 

 their increase in length is brought about not by direct forward 

 growth of the first formed parts of the tubes, but by the 



