470 



MISS K. M. PARKER ON THE 



Comparison with the succeeding stages shows that this repre- 

 sents the ventral portion of the future second aortic arch (Pis. I., 

 II., figs. 4 & 8, ^ 2), while a small vessel arising from the dorsal 

 aorta and running outwards on the dorsal wall of the gut ventral 

 to the auditory vesicle, corresponds with the dorsal portion of the 

 completed arch in later stages. 



Text-figure 9. 





Perameles nasuta (1 S, A.). 



A. Transverse section in region of lateral heart primordia and open gut. 



B. Complete outline of same section. 



A'i. Second aortic arch. D.A. Dorsal aorta. ISnd. Endothelium of heart. 

 M.F. Medullary fold. My. M3'ocardial layer. P. Pericardium. V.C.M. Vena 

 capitis medialis. F.P.I. First visceral pouch. 



In the median space between the anterior ends of the endo- 

 thelial heart-tubes are a number of scattered angioblast cells 

 lying between the splanchnic mesoderm and the entoderm 

 (PI. I. fig. 4 and text-fig. 7). These cells possibly represent the 

 primordia of the capillaries found in the corresponding position 

 in the next stage. They aSbrd an instance of the origin of 

 angioblast cells from the splanchnic mesoderm after the establish- 

 ment of the definitive endothelial heart-tubes. 



Posterior to the region represented in fig. 4, the heart-tubes 



