DEVELOPMENT OF THE HEART IN MARSUPIALS. 485 



third pairs of aortic arches arise from the median ventral aorta 

 immediately posterior to its bifurcation. The second arch is 

 large, the third very small. 



In correlation with the rapid development of the fore- and mid- 

 brains at this stage, the head-plexus found in Stage Y. (see PI. II. 

 figs. 6-8) has become extended into a long slender vessel, destined 

 to form the anterior part of the internal carotid artery and lying 

 on either side of the mid-ventral line in the fore-brain region. 

 As in the preceding stage, it anastomoses anteriorly with capillaries 

 arising from the vena capitis medialis. 



Both venffi capitis medialis (text-fig. 22, V.CJL) and lateralis 

 are present, though neither can be traced continuously throughout 

 the head-region. The vena capitis medialis extends to the ante- 

 rior end of tlie brain, lying close to the medullary tube,_ dorsal to 

 the dorsal aorta and the internal carotid artery. It is discon- 

 tinuous in the region of the auditory vesicle, where no veins are 

 recognisable. Posterior to that, it runs back as a continuous 

 trunk to the level of the Cuvierian duct, and beyond this is 

 recognisable as a minute vessel lying close to the neural tube in 

 the trunk region. 



Lying lateral to the primordium of the trigeminal nerve, there 

 are a few scattered capillaries which represent discontinuous 

 segments of the vena, capitis lateralis. Immediately posterioi: to 

 the primordium of the trigeminus, the vena capitis lateralis arises 

 from the vena capitis medialis and runs back as a small vessel 

 lying lateral to the root of the facial nerve. Vense capitis lateralis 

 and medialis are interrupted in the region of the auditory vesicle, 

 but both are present immediately posterior to it. At irregular 

 intervals on their course there are transverse communications 

 between the two veins. The vena capitis lateralis does not foi'm 

 a continuous trunk in the region posterior to the auditory vesicle,, 

 but immediately anterior to the point of separation of right and 

 left heart-tubes it increases markedly in size and is connected 

 by a wide anastomosis with the vena capitis medialis, which 

 becomes very small posterior to this level. The enlarged vena 

 capitis lateralis, or, as it may here be called, anterior cardinal 

 vein, passes ventro-laterally and, running alongside the dorsal 

 aorta for a short distance, finally opens into the Cuvierian duct 

 in the manner seen in text-fig. 22. 



The umbilical vein is now present, running in the somatopleure 

 and opening into the Cuvierian duct. 



In this stage, then, we have a heart in which fusion of tlie 

 right and left primordia has occurred except in the region of the 

 sinus venosus, and curvature has carried the auricular limb into 

 position dorsal to the ventricle. Three aortic arches are present^ 

 ven^ capitis medialis and lateralis are well established though 

 discontinuous and open via the Cuvierian ducts into the sinus 

 venosus. 



