468 



MISS K. M. PARKER ON THE 



as well as the topography of the endothelial heart-tubes and the 

 aortse and aortic arches, will be understood best by reference 

 to fig. 4 and text-fig. 12. From fig. 4 it will be seen that the 

 pericardium (P.) has increased markedly in size. As in the 

 previous stage, its inner wall is closely applied to the entoderm 

 of the crescentic or U-shaped anterior intestinal portion [A. I. P.) 

 {cf. text-fig. 12). The antero-median portion of the pericardium 

 is somewhat rectangular in transverse section (text-fig. 7) ; its- 

 dorsal wall is slightly thicker than its ventral, and between 

 the former and the fioor of the gut lie the endothelial heart- 

 tubes (text-fig. 8) and the first aortic arches (text-fig. 7). 



Text-figure 7. 



Ferameles nasvta (1 S, A.). Transverse section through median 

 pericardium and first aortic arch. 



^1. First aortic arch. B.A. Dorsal aorta. F.&. Fore-gut. M.P. Medullary 

 plate. M.m.A. Maxillo-mandibular arch. N. Notochord. P. Pericardium.. 

 V.C.M. Vena capitis medialis. 



In the antero-median limb of the pericardium, the endo- 

 thelial heart-tubes, enclosed in a fold of splanchnic mesoderm, 

 lie separate from each other below the closed gut (PI. I. fig. 4 

 and text-fig. 8), while at the level of the anterior intestinal 

 portal they diverge in the manner indicated in fig. 4 and lie on 

 either side of the open gut in the dorso-medial wall of the peri- 

 cardium (text-fig. 9). In their posterior portions the heart 

 primordia lie on the ventral side of the pericardium. 



The endothelial heart-tubes throughout their length are almost 

 completely enclosed by the layer of splanchnic mesoderm con- 

 stituting the primordium of the myocardium. In the posterior 

 region of the heart the myocardial layer is closely applied to the 

 outside of the endothelial tube (text-tig. 10), but in the greater 

 part of its extent there is a considerable space between the two; 

 layers of the heart primordium (text-fig. 9). 



From the cephalic extremity of each endothelial tube, there 

 arise two vessels, one of which runs forwards and outwards; 



