DEVELOPMENT OF ARTERIES 217 



stage when its obliquely directed upper free edge bounded a relatively large 

 interventricular foramen. This becomes constricted by the fusion of the dorsal horn 

 of the septum with the already fused endocardial cushions of the auricular canal. 

 This fusion takes place nearer the right than the left edge of the cushions (fig. 268), 

 so that the septum is placed nearer the right than the left auriculo- ventricular 

 opening. The two ventricles properly so-called are now completely separated, but 

 a foramen still connects the left chamber with the distal part of the right ventricle 

 or proximal chamber of the aortic bulb, meanwhile divided by the fusion of its two 

 endocardial ridges with one another, and with the aortic septum proper. The 

 septum of the bulb now unites with the remaining free edge of the interventricular 

 septum, the pars membranacea septi is formed, and the two sides of the heart are 

 finally entirely isolated. 



DEVELOPMENT OF THE ARTERIES. 



The earliest stages in the development of the vessels have been already described 

 (p. 63). We shall here begin with a stage reached in the human embryo about 

 the fifteenth day. 



Dorsal aorta (fig. 270). The two dorsal aortse of the early stages come 

 together in the middle of their course, and fuse into a single mesial vessel. The fusion 

 proceeds forwards and backwards, but at the head end the primitive arteries remain 

 separate, one on either side, on the dorsal aspect of the pharynx. Behind, the 

 vessel divides into the allantoic arteries, which are continued into the body-stalk. 

 These vessels correspond to branches of the aorta which vascularise the allantois in 

 lower Amniota, and which appear at a later stage after the yolk-circulation has 

 been established. 



Aortic arches. By the term aortic arches we understand a series of vascular 

 loops which are formed in the branchial arches, and join the ventral aortse to the 

 dorsal aortse round the walls of the pharynx. They appear in succession from 

 before backwards, one in each branchial arch. On the thirteenth day the primitive 

 mandibular loop is alone present ; by the fifteenth day a second arch is completed 

 in the hyoid arch (fig. 270) ; and by the eighteenth day three others have been 

 added (fig. 271). The first three, along with the ventral and dorsal vessels, by a 

 series of changes presently to be described, form the carotid system of arteries, the 

 fourth becomes the systemic arch, and the last the pulmonary arch. Between the 

 systemic and pulmonary arches a vessel appears at a rather later stage ; this has 

 been regarded as a rudimentary fifth arch corresponding to the fifth arch of lower 

 forms. It soon disappears, and takes no share in the formation of any adult 

 vessels (Zimmermann and Tandler). 1 If this be so, the pulmonary arch must be 

 the sixth of the series. 



The observations on the fifth arch in mammalian embryos are not quite in complete accord, 

 but the variations in development of the vessel, interpreted in this sense, may perhaps be due to 

 its transitory and vestigial character ; and in view of the conditions prevailing in lower forms 

 the conclusion that there is a rudimentary fifth arch in mammals also, in front of the 

 pulmonary, seems justified on the evidence afforded by recent research. - 



In the majority of cases the anterior arches have already become incomplete before the 

 last has been formed, but in the human embryo the series (with the exception of the 

 rudimentary fifth arch) is complete for a time. 



1 Tandler, Morph. Jahrb. xxx. 



2 Zimmermann (Verb. Anat. Ges. 1887) first showed the existence of a vascular channel between 

 the aortic and pulmonary arches in the human embryo. It took the form of a vessel springing from 

 the systemic arch and joining it again. Tandler (loc. cit.) described a more complete arch (figs. 272, 

 278) passing from the systemic to the pulmonary arch. For detail as to the lower mammals, see 

 Zimmermann, Anat. Anzeiger, iv. 1889 ; Lehmann, ibid. xxvi. 1905, and Zool. Jahrb. xxii. ; Locy, 

 Anat. Anzeiger, xxix. 1906 ; Lewis, ibid, xxviii. 1906. The last named holds it doubtful whether the 

 irregular channels described as fifth arches are really of that nature. 



