DEVELOPMENT OF THE HEART IN MARSUPIALS. 



471 



gradually curve outwards and pass imperceptibly into vitelline 

 veins. 



The dorsal aorta is paired and runs back continuously, the two 

 vessels keeping approximately the same distance from the middle 

 line (PI. I. fig. 4, D.A.). 



Immediately dorsal to the dorsal aorta on each side there is 

 situated a series of apparently isolated sections of a minute blood- 

 vessel (text-fig. 7, V.C.M.). These capillaries lie close against 

 the medullary tube, medially to the neural crest proliferation in 

 the region of the trigeminal ganglionic primordium. From the 

 position of this vessel relative to the dorsal aorta and nerves, 

 it is evidently the vena capitis medialis of Grosser (5). A brief 

 summary of some of the literatui-e on the subject of the vense 

 capitis medialis and lateralis and their relations to the anterior 

 cardinals will be found below, together with a review of the facts 

 of development of these veins in Perarneles. 



Text-figure 10. 



Perameles nasuta (1 S, A.). Transverse section through posterior 

 portion of lateral heai't primordia. 



B.A. Dorsal aorta. End. Endothelium of heart. M.F. Medullary fold. 

 Ily. Mj'ocardial layer. 



Although it is not within the scojoe of this paper to deal 

 in any detail with the posterior vessels, it may be noted here that 

 in this stage, in the region of the posterior somites, there occur 

 intersegmental oflTshoots from the dorsal aortas. These inter- 

 segmental offshoots are shown very clearly in the longitudinal 

 series (text-fig. 11). It will be seen from the figure that between 

 each two successive somites (^S.) there is a small dorsal offshoot 

 from the aorta. Dorsal to the somites there are a few scattered 

 endothelial cells {End.). The longitudinal vessel connecting the 

 offshoots is not continuous, but portions of it are piesent in the 

 next section. 



At the level of the second somite there occurs a pair of small 

 vessels lying in the somatopleure immediately dorsal to the heart- 

 tubes. Each consists of a single vessel with a few minute 



