68 



HUMAN EMBEYOLOGY. 



External carotids 



right subclavian is developed from the seventh right somatic inter-segmental artery. 

 The ventral root of the left fourth arch and the arch itself form the arch of the aorta. 

 The fifth arch, as already stated, is quite transitory ; it leaves no remains, therefore 

 it is not necessary to speak of any part of the ventral stem as its ventral root. The 

 ventral part of the sixth arch on the right side becomes the extra-pulmonary part 

 of the corresponding pulmonary artery. On the left side it practically disappears. 

 On the right side the dorsal part disappears, but on the left side it persists, till birth, 

 as the ductus arteriosus, which connects the pulmonary artery with the aorta, and 

 after birth it is converted into the ligamentum arteriosum. The truncus arteriosus 

 is cleft into two parts by a spiral septum ; one part, which remains continuous with 



the ventral roots of the fourth arches 

 and therefore with the innominate artery 

 and the aortic arch, becomes the ascend- 

 ing aorta, and the other, which remains 

 associated with the sixth arches, becomes 

 the stem of the pulmonary artery. 



Whilst the changes mentioned have 

 been taking place in the cephalic part 

 of the arterial system, the primitive 

 dorsal aortae have fused together from 

 a point immediately caudal to their 

 seventh dorsal branches to a point 

 immediately cranial to the origins of 

 ,imn Story the umbilical branches from their ven- 

 Arch of aorta tral aspects, and their ventral branches 

 have fused together into single stems, 

 some of which have been converted into 

 the cceliac, the superior mesenteric, and 

 the inferior mesenteric arteries. The 

 fusion of the dorsal aortae commences 

 in embryos about 2'5 mm. long. When 

 the embryo has attained a length of 

 5 mm. the fusion has extended to the 

 caudal ends of the aortae and the single 

 stem is continued into the rudimentary 

 tail as the caudal artery, which after- 

 wards becomes the middle sacral artery. 

 After the fusion is completed the um- 

 bilical arteries spring from the ventral 



aspect of the single dorsal aorta. (For 

 iiG. 87. SCHEMA OF PART OF THE VASCULAR SYSTEM , i r f ,-, , . , , , . -, 



OP A FCETUS SEEN FROM THE FRONT. Showing the the ^thei history of the arterial system, 



origin of the positions of the first and second arches, S66 under Vascular System.) 



the dorsal roots of the third arches on both sides, The Primitive Venous System. The 



and the dorsal roots of the fourth and fifth arches n i n , 



on the right side are shown in dotted lines. The nrst definite venOUS trunks to appear 



positions of the fifth arches are not shown. are the umbilical veins, returning blood 



from the chorion, and the vitelline 

 veins, which convey the blood from the yolk-sac. 



Two umbilical veins enter the body-stalk and unite to form the vena umbilicalis 

 impar (Fig. 81). This divides, at the posterior margin of the umbilicus, into the 

 right and left lateral umbilical veins, which run round the lateral margins of the 

 umbilical orifice, in the lateral margins of the body wall of the embryo. At 

 the cranial margin of the umbilicus they turn medially, enter a transverse bar of 

 mesoderm which forms the caudal boundary of the pericardium and is known as 

 the septum transversum, and pass through it into the caudal end of the heart 

 (Figs. 81, 82). 



The blood from the yolk-sac passes, for a short time, into the lateral umbilical 

 veins at the posterior margin of the umbilicus. This is a very transitory arrange- 

 ment, ancf it is soon replaced by the formation of two proper vitelline veins, one 

 on each side, which ascend, along the vitello-intestinal duct, to the cranial margin 



1st arch- 



2nd arch, 



Internal carotid 



Internal carotid 



Internal carotid 

 Right common carotid 

 Dorsal root of 3rd arch"". 

 Right sub-_ _ 

 clavian artery 

 Innominate artery" 



Right pulmonary" 1 !" 

 artery ,', 

 Ascending aorta" ri~~ 



Pulmonary artery 



Dorsal root of. 

 right 6th arch 



Internal carotid 



__ External 

 carotid 



External 

 * carotid 



. Dorsal root of 

 ,S left 3rd arch 

 Left common 

 V " carotid 

 V, Arch of aorta 

 th arch) 



\Ductus 

 arteriosus 



\ Arch of 

 aorta 



X Left pulmon- 

 ary artery 

 Dorsal aorta 



-- Dorsal aorta 



