852 



DEVELOPMENT 



[CH. LIX. 



ci 



with the caudal arch, becomes the internal iliac or hypogastric artery. 

 This is continued in the embryo along the ventral wall of the body as 



the umbilical arteries to the placenta. 

 The Heart. The simple longi- 

 tudinal heart soon becomes separated 

 by three transverse constrictions into 

 four chambers, which are, from behind 

 forwards, the sinus venosus, the 

 auricle, the ventricle, and the aortic 

 bulb (figs. 648 and 649). The sinus 

 venosus receives the omphalo-mes- 

 enteric and other veins, and the aortic 

 bulb terminates in the fifth arches 

 and the ventral roots of the fourth 

 arches. The sinus venosus is gradu- 

 ally absorbed into the auricle, and at 

 the same time the heart tube bends 

 so that the auricle is placed behind 

 the ventricle and the aortic bulb 

 that is, between them and the wall 

 of the foregut (figs. 650 and 651). 

 As soon as the bending is completed 

 each chamber is divided by septa 

 into right and left halves, but an 

 opening, the foramen ovale, remains 

 in the interauricular septum till after 

 birth. The aortic bulb is also divided 

 into two parts : one of these is con- 

 nected above with the fifth arches, 

 which become the pulmonary arteries, 

 and below with the right ventricle; 

 it becomes, therefore, the stem of 

 the pulmonary artery. The other 

 part, which is connected with the 

 roots of the fourth arches above and 

 the left ventricle below, forms the 

 ascending part of the aorta. 



The Veins. 1. The veins of 

 the embryo are the omphalo-mes- 

 enteric, which carry blood from 

 the yolk - sac to the heart. 2. 



The umbilical or allantoic, bearing oxygenated blood from the 

 placenta to the heart. 3. The primitive jugular veins, one on each 

 side returning blood from the head, neck, and upper extremities. 

 4. The cardinal veins returning blood from the body walls, the 



pn 



FIG. 647. Diagram of the aortic arches in a 

 mammal, showing transformations which 

 give rise to the permanent arterial vessels. 

 A, Primitive arterial stem or aortic bulb, 

 now divided into A, the ascending part of 

 the aortic arch, and p, the pulmonary; 

 a a', right and left aortic roots; A', de- 

 scending aorta ; 1, 2, 3, 4, 5, the five primi- 

 tive aortic or branchial arches ; 7, //, 7/7, 

 IV, the four branchial clefts which, for the 

 sake of clearness, have been omitted on the 

 right side. The permanent systemic vessels 

 are deeply, the pulmonary arteries lightly, 

 shaded; the parts of the primitive arches 

 which are transitory are simply outlined; 

 c, placed between the permanent common 

 carotid arteries ; c e, external carotid arte- 

 ries ; ci, internal carotid arteries ; s, right 

 subclavian, rising from the right aortic root 

 beyond the fifth arch; v, right vertebral 

 from the same, opposite the fourth arch; 

 v' s', left vertebral and subclavian arteries 

 rising together from the left, or permanent 

 aortic root, opposite the fourth arch; 

 p, pulmonary arteries rising together from 

 the left fifth arch ; d, outer or back part of 

 left fifth arch, forming ductus arteriosus; 

 p n, p n', right and left pneumogastric 

 nerves descending in front of aortic arch, 

 with their recurrent branches represented 

 diagrammatically as passing behind, to illus- 

 trate the relations of these nerves respec- 

 tively to the right subclavian artery (4) and 

 the arch of the aorta and ductus arteri- 

 osus (d). (Allen Thomson, after Rathke.) 



