DEVELOPMENT OF THE HEART IN MARSUPIALS. 



481 



and ventral to the gut. The vena capitis medialis runs back 

 alone for a short distance, and is then again connected with a 

 more laterally situated capillary. This capillary runs back from 

 this point to the level of the incipient third aortic arch, and then 

 runs latei'ally to be connected with the Cuvieiian duct which is 

 now well developed (fig. 7 & text-fig. 19 J. On the right side of 

 the embryo this transverse connection is very incomplete, but not 

 quite so incomplete as would appear from the model, since the 

 diflSculty of building up these fine capillaries caused some to be 

 lost in this region. This vein, lying lateral to the vena capitis 

 medialis and to the primordia of the nerve-ganglia, is the vena 

 capitis lateralis of Grosser (6) and Salzer (17). From the vensB' 

 capitis medialis and lateralis the anterior cardinal vein is derived. 

 The auditory vesicle lies in the space between the venfe capitis 

 medialis and lateralis immediately dorsal to the second aortic 

 arch. Running in thehyoid arch area few capillaries apparently 

 corresponding with the much more conspicuous group in the 

 mandibular arch. 



Text-figure 20. 



Kci 



Ferameles ohesula (I0.viii.03). Transverse section showing the connection of the 

 vena capitis medialis with tlie dorsal aorta. 



D.A. Dorsal aorta. 



M.F. Medullary plate. V.C.M. Vena capitis medialis. 

 V.C.L. Vena capitis lateralis. 



The Cuvierian ducts have increased considerably in size, and 

 that of the left side opens direct into the lateral heart-tube (text- 

 fig. 19, CD.). Behind the opening of the Cuvierian duct a single 

 small capillary runs posteriorly in the somatopleure representing 

 the future umbilical vein. The vena capitis medialis, it should 

 be noted, continues on after the lateral bend of the vena capitis 

 lateralis (fig. 7). The two are closely connected in the region of 

 the incipient third aortic arch. 



One important point which is dijQficult to observe in the figures 

 of the model is shown in the sections of the embryo (text-fig. 20), 

 and that is the fact that the vena capitis medialis at irregular 

 intervals opens into the dorsal aorta. 



In this stage, then, we find the ventricular and auricular 



