398 COMPARATIVE ANATOMY 



cardinal vein from the body generally (not including the alimentary 

 canal). The caudal vein, which lies directly beneath the caudal aorta, 

 is connected with the posterior cardinals, usually indirectly, through 

 the renal portal veins (cf. Figs. 325-329). The further development 

 of the embryonic vessels may take place in one of three ways. 



(1) The embryo may leave the egg, and take on an aquatic 

 existence (Anamnia), making use of its branchial vessels for pur- 

 poses of respiration, the entire allantois (Amphibia) when present 

 giving rise to the bladder. 



(2) In the Amniota, which from the first breathe by means of 

 lungs, a modification and reduction of the branchial vessels and 

 allantois takes place, and the latter may even disappear entirely. 



(3) The embryo undergoes a longer intra-uterine existence, 

 the allantois coming into close connection with the walls of the 

 uterus by means of the chorionic villi: the allantoic vessels extend 

 into the wall of the uterus and come into more or less close relations 

 with the maternal vessels, thus serving for the respiration and 

 nutrition of the foetus. In this way a placenta and a placental 

 circulation arise (pp. 9 11). 



On the appearance of pulmonary respiration, important changes 

 take place in the branchial vessels and heart. The formation of a 

 septum both in the atrium and in the ventricle leads to the presence 

 of two atria or auricles and two ventricles, the conus arteriosus 

 and sinus venosus becoming eventually more or less incorporated in 

 the right ventricle and auricle respectively. Thus a right (venous) 

 and a left (arterial) half can be distinguished ; and a new vessel, 

 the pulmonary artery, arising from the last arterial arch, becomes 

 connected with the right ventricle ; this conveys venous blood to the 

 lungs, while special vessels (pulmonary veins) return the oxygenated 

 blood from the lungs to the left auricle, from which it passes into 

 the left ventricle and so through the aorta into the general 

 circulation of the body. 



The branchial vessels never become functional, as such, in any 

 period of development either in Sauropsida or Mammalia, but those 

 which persist give rise, as already mentioned, to important vascular 

 trunks of the head and neck (carotids), anterior extremities (sub- 

 clavians), and lungs (pulmonary arteries), and also to the roots of 

 the aorta, one or both of which may remain (cf. Fig. 301). 



The primitive number of arterial arches is six, the first two of 

 which (belonging to the mandibular and hyoid arches respectively) 

 almost always disappear early (Fig. 301): in caducibranchiate 

 Amphibia (including Anura) and in Amniota, the fifth arch also 

 disappears. The third gives rise to the carotid arch ; the fourth of 

 both sides (Amphibia, Reptilia), or of one side (Aves, Mammalia), 

 to the aortic or systemic arch, and the sixth to the pulmonary arch. 



From the Dipnoi onwards, the posterior cardinals become more 

 or less completely replaced functionally by a large unpaired vein, 

 the postcaval or posterior vena cava, which opens independently into 

 the right auricle. 



