482 



MISS K, M. PARKER ON THE 



portions of the heart differentiated. Fusion of right and left 

 primordia has only affected the myocardium and the cephalic ends 

 of the endothelial tubes. The two halves of the auricular portion 

 of the heart are wide aj)art. 



Two complete aortic arches are present and one is in process of 

 formation. Yenae capitis medialis and lateralis and Ouvierian 

 ducts are all present. 



It is not necessary to give a detailed description of the embryo 

 of Macropus sp., which is included in this stage, as it differs only 

 in certain points from the Perameles ohesula embryo described 

 above. In the degree of development of the gut and pharyngeal 

 pouches, as well as of the nervous system, the two embryos very 

 closely resemble each other. 



^.C.L 



Text-figure 21. 



V.C.M. _^M.P 



Macropus sp. Transverse section througli the root of the second aortic arch. 



D.^. Dorsal aorta. JEwi. Endothelium. Jlf.P. Medullary plate. Ji";^. Myocardium. 

 Rt.A. Root of aortic arch. T.Gr. Primordium of trigeminal ganglion. 

 V.C.L. Vena capitis lateralis. V.C.M. Vena capitis medialis. 



Two complete aortic arches are present, but there is no trace of 

 a third. 



The vena capitis medialis resembles that of the Perameles 

 embryo. The vena capitis lateralis, however, is slightly less 

 advanced, being only recognisable in the region of the trigeminal 

 and facial neural crest proliferations, and not extending back as 

 far as the auditory vesicle. Ouvierian ducts are present, and the 

 right one at least opens into the heart-tube. The sections are 

 somewhat broken in this region, so that satisfactory observations 

 on the openings of the Ouvierian ducts and their relations to 

 the anterior cardinal veins are impossible. 



The heart diffei^s in several respects from that of the Perameles 

 embryo. The genera] relations of heart and pericardium and the 

 mode of origin of the aortic arches are exactly the same in the 



